Aquarium Terms & Definitions
 We've compiled a list of terms commonly used in the aquarium industry or hobbyist community - it can be a lot to figure out, especially when starting out!
Search by letter, or type Ctrl+F to open a search field and find what you're looking for. Links in this article link to other definitions, not to products in our store. (Is something missing? Let us know!)
actinic light
Actinic light comes from the blue end of the color spectrum, so it appears blue and is usually combined it with a full spectrum (white) lamp for clear viewing. Blue light travels furthest in the ocean, and is essential to photosynthesis in zooaxanthellae algae, a symbiotic algae which photosynthetic corals and invertebrates need to live.
activated carbon
A form of carbon derived mostly from charcoal with ideal surface area and porosity for the adsorption of chemical compounds from your water, like medicines, odors or discoloration. Carbon must be changed every 1-2 months, to avoid leaching impurities back into the water.
adsorption
If absorption is when a substance is soaked up by another - think of a
sponge - then adsorption would be if the sponge accumulated liquid
around itself to form an outer membrane. Chemical filter media
removes waste by adsorbing it.
aeration
Refers to the injection and distribution of air into your tank, either by gas exchange at the water's surface, an air pump or powerhead with venturi jet. The purpose of aeration is to supply water with enough dissolved oxygen to sustain life.
air pump
A pump that injects air into your aquarium, to drive sponge filters or under gravel filters, protein skimmers, air stones, etc. Many filters introduce some amount of oxygen to your water, but air pumps are important to proper aeration in larger/heavily stocked tanks. High pressure air pumps are available for really large/deep tanks.
air stone
Block of porous limestone or wood which, when air is forced through it via
an air pump, diffuses a stream of tiny micro-bubbles that create a
dynamic visual effect and aid in oxygenation. Used in air-driven protein
skimmers as well. Air stones slow down as they get clogged, so clean or
replace them when this happens.
algae
These plant-like organisms grow whenever they have water, light and nutrients. There are many types - from annoyingly resilient green or brown slime, to macroalgae that resemble normal plants, to coralline algae which is an attractive accent in reef tanks. Some things commonly thought of as algae are actually cyanobacteria, like the "blue-green algae" spirulina.
algal filter
One approach to reducing nitrates in a saltwater tank. An algal filter uses macroalgae, which, like all plants, consume nitrates. Algal filters are usually housed in a sump chamber, since macroalgae grow quickly.
algicide
Chemical treatment that kills big algae blooms quickly. They will not treat the root cause of algae problems - too much sunlight or the wrong spectrum of lighting, too many nutrients from overfeeding, phosphates or silicates in water or high nitrates - and could cause more complications by overloading your filters with a heavy wasteload of dead algae. Never use in tanks with live plants or invertebrates.
alkalinity
A measure of your water's resistance to changes in pH (or "buffering capacity"). Commonly measured as carbonate alkalinity (kH) or total alkalinity (gH). Alkalinity can be raised by adding a buffer when you condition your water.
ammonia
NH3. Produced as the first waste gas in the nitrogen cycle from decomposing food or fish poop. It is toxic to most creatures in very small amounts until converted into nitrates by your biological filter. You should not be able to detect ammonia in a healthy, functioning and cycled tank.
aerobic
Describes an organism or process that consumes oxygen to function.
anaerobic
"Without air". Refers to an area with no dissolved oxygen - these can build in pockets between rock without adequate circulation, or under fine grained substrates unless you have a burrowing organism to stir it up. Anaerobic bacteria are cultivated in some types of systems like deep sand beds, because they are able to convert nitrates into harmless nitrogen gas. However, in most systems anaerobic bacteria are not desirable; they produce hydrogen sulfide and other toxins.
aquaculture
Refers to the farming or "culturing" of aquatic organisms, either within existing bodies of water or in isolation. When done ethically, this is a practice which can prolong the life and health of the world's natural coral reefs and aquatic ecosystems.
aragonite
A form of calcium carbonate that makes up coral skeletons; is found in reef sand and coral substrates. Aragonite can help to harden your water/raise pH.
artemia
Scientific name for brine shrimp.
ball valve
A simple, durable shutoff valve design used in flow control. Especially
useful in adjusting high pressure flows such as those from pumps or canister filter returns.
ballast
Any type of fluorescent lamp or metal halide lamp must be plugged into a ballast before a power source, because gases in the bulbs are unstable and they can actually explode without a ballast to ground them. Ballasts are often built into fixtures; if buying one separately, it must match to the type of bulb you are using (a metal halide ballast must be for either single ended or HQI bulbs, and must match the wattage). Different types of fluorescent lamps may not be compatible in the same fixture, such as Very High Output lamps vs. standard fluorescents.
Berlin filter
Low maintenance wet/dry filter design invented in Germany. Extremely minimal and natural, it is quite popular with reef aquarists. It consists of a sump full of live rock as the biological filter, lighting to keep the organisms on the rock alive, and a protein skimmer for mechanical filtration.
bioload / bio-load
Term that describes how much waste is produced by the organisms in your aquarium. The heavier your bioload, the more filtration & circulation you need. Some fish create disproportionately large bioloads, like goldfish and Oscars. Your aquarium's capacity to handle a bioload is limited by its size, which is why you shouldn't overstock a tank.
biological filtration
What we call biological filters are just housing - their main function is to maximize surface area for colonization by billions of helpful bacteria that do the actual filtering itself. These bacteria are the only way to break down naturally occuring, toxic wastes in a process called the nitrogen cycle. Biological filtration is the most important type and without it, your aquarium is uninhabitable. Biological filters include sponge filters, under gravel filters, wet/dry filters and even live rock, though most filter styles offer it in some capacity.
blackwater
Really a clear, light yellowish-brown, "black water" refers to conditions in
rivers like the Amazon which flow through rich vegetation (e.g. a
rainforest). Plants at the water's edge leak "tannins" into the water - the
same thing that dyes your water when you make tea. Tannins
are acidic and create the soft water conditions preferred by Amazon
fishes.
bottom feeders
Organisms like catfish, crustaceans and mollusks are usually bottom
feeders - they scavenge or feed on things that live on the ground. Food
for bottom feeders is designed to sink quickly.
brackish
Brackish water is a mix of fresh water and salty ocean water that occurs in the mouths of many rivers (these areas are called estuaries) as well as mangrove swamps and certain lakes and small oceans, like the Baltic. The salinity of these habitats changes with the tide, so brackish water species are a lot hardier than most. Many fish spawn or spend part of their lives in brackish conditions.
brine shrimp
Brine shrimp are a high fat, tiny organism that is a great food for fry and a nice occasional treat for freshwater fish. They do not contain the right kind of fat to sustain marine organisms and so should not be fed to them unless they are "gut-loaded", or enriched by a diet of appropriate fats. Can be easily hatched as live food and eggs, if dry, stay good for years. Bizarrely, many of us first encountered these as "sea monkeys".
bubble wand
A bubble wand is powered by an air pump and
creates striking displays of tiny bubbles along the length of the wand. Unlike an air stone, a
bubble wand makes a "curtain" of bubbles that can
be shaped to follow edges or frame objects in your tank. Purely for
decoration and a mild boost to gas exchange.
buffer
A "buffer" or buffering solution is essentially a solid dissolved in water that boosts the alkalinity ("hardens") or adjust the pH. Examples include calcium carbonate. Buffer is also a verb: the more dissolved solids, the more your water can "buffer" or stabilize against small changes in water conditions.
calcium
Undoubtedly the most important trace element on the reef, dissolved calcium is used to create the striking skeletal structures of mollusks and corals that form the backbone and beauty of these habitats. Ocean water contains about 410 ppm of this mineral the world over - it's actually supersaturated, meaning, there's more calcium than water should actually be able to hold, due to the presence of mediating factors (most importantly, magnesium). Some form of calcium must be added to saltwater tanks, usually with a reactor, and replenished as depleted.
calcium carbonate
CaCO3. Calcium in sea water precipitates (becomes a solid) as calcium carbonate, which forms the shells of mollusks and the skeletons of corals and invertebrates.
calcium hydroxide
Ca(OH)2. See kalkwasser.
canister filter
The canister filter is completely external to the aquarium, unlike most filters which hang on the tank or are submerged. It consists of one or more "canisters" filled with filter media - because tank size doesn't matter, a canister can be much larger than traditional filters and provide more surface area for better filtration. A water pump (usually built-in) forces pressurized water through the canister.
carbon
See activated carbon
carbon dioxide
Carbon dioxide (CO2) is vital to plants. Injection of co2 (using CO2 reactors or DIY systems) also softens water. Plants release oxygen during daylight hours, but at night they produce carbon dioxide, so there's no need to add co2 at night (and using an air pump during those hours may be a good idea too).
carnivore
An organism that eats the flesh of other organisms to survive, either through predation or scavenging.
check valve
An automatic valve that allows liquid or gas to flow through in one
direction only. Important to aquarium plumbing as a well placed check valve can help avoid overflow or back-siphoning.
chemical filtration
Chemicals are removed from your water with activated carbon or ion-exchanging resins in your filter. You may only need chemical filtration if you've used medicines or experience unwanted odors, colors or contaminants in your water like silicates or phosphates. Chemical filter resins usually need to be exchanged or removed within a month or 2 to avoid leaching chemicals back into your water.
chiller
A pricey device that cools down your water. Chillers may be necessary if you have a lot of submerged equipment raising your water temperature, if you're using metal halide lighting, or if you live in a warm climate but keep cold water organisms.
chloramine
Sometimes present in tapwater, where it's added to kill bacteria. Supposedly harmless to humans, but is poisonous to fish and kills good bacteria. Chloramine won't evaporate from water, but chlorine removers will get rid of it.
chlorine
Like chloramine but more widely used, chlorine is added to tapwater to kill bacteria. It's poisonous to fish and good bacteria, but can be removed by dechlorinators, or by letting water sit out for 24 hours.
cichlids
An incredibly diverse family of fresh and saltwater fish from South America and Africa. Characterized by striking shapes, colors, and personalities, many are aggressive and shouldn't be combined with others. Common aquarium cichlids include freshwater angelfish, Oscars, and discus. Rose to popularity as freshwater aquarists sought further challenges, since these fish often have very specific needs.
circulation
Proper circulation is the healthy movement of your entire volume of water, quickly enough to distribute trace elements and oxygen throughout the entire tank. Poor circulation is incomplete or inadequate movement that deprives areas of your tank of essential nutrients or elements, as well as reducing the efficiency of filters, heaters and other devices that rely on water flow. Water pumps increase circulation (many filters include pumps).
clarifier
See water clarifier.
community tank
Fish tank housing multiple species of organisms that can live together peacefully.
conditioner
A conditioner is used to prepare tap water or filtered water before you add it to your aquarium. Water conditioners may remove chlorine and heavy metals in tap water, they may boost your fish's slime coat, or they may try to approximate special conditions, like Amazon blackwater conditioners.
coral propagation
See fragging.
coralline algae
An encrusting pink-purple algae that is a prized member of the reef tank and a main food source for some creatures, including sea urchins and parrotfish. (It may appear in other colors, but is typically reddish.) Calcium deposits from sea water make this algae hard. Coralline algae will encrust rock and the walls of your tank, and makes a lovely display on the back and sides of your tank. It needs actinic light to thrive.
current
Current describes flow (water currents, electrical currents). Natural water current patterns are important to consider when designing healthy circulation for your aquarium - particularly in marine tanks, where natural currents are quite vigorous. Powerheads are primarily used to recreate current patterns.
cycling
Process of establishing a complete nitrogen cycle in your tank. Before a tank cycles, it contains free toxic agents like ammonia and nitrite that kill most organisms and cause discomfort and shortened life cycles for others. Cycling can take up to two months, but can be aided by natural cycling aids.
cycling aid
An additive meant to speed up cycling in an aquarium. You will not be able to tell it's working without testing for ammonia, nitrites and nitrates at every step of the cycling process. Nitrites must spike from 0, and return to 0, before the cycle is complete. Natural cycle aids include established biological media from other tanks, or live plants.
deionization / deionizer
A process for filtering tap water of impurities before adding to the aquarium. Deionization removes important trace elements that should be replaced before adding to your aquarium. Deionizers are often combined with reverse osmosis filters.
deep sand bed
A technique used to reduce nitrates in saltwater aquaria, a DSB is 4"-6" of very fine grained sand. Sand stirring organisms like burrowing mollusks or fish keep the top layers oxygenated, while the depth of the bed ensures that its deepest layer receives no oxygen, thereby allowing anaerobic bacteria to grow. These bacteria can convert nitrates into harmless nitrogen gas, one of few ways to remove nitrates from water. Care must be taken with the DSB because of toxic gases also produced by these bacteria. Several variations exist, including the Plenum sand bed and remote deep sand bed (RDSB; kept outside aquarium, in a sump).
diatom filter
Specialized mechanical filter that uses diatomaceous earth to remove very fine particles from the water. They are the most effective mechanical filters, but because of this they would clog constantly if used all the time. They're used for regular maintenance to polish (clarify) water - regular use is proven to remove hard to kill disease agents for healthier water.
diatomaceous earth
Sediment composed of fossilized diatoms; appears to be a very fine powder. Its porosity makes it a uniquely powerful filter media but take care not to inhale it or it can cause respiratory problems. Used in all sorts of commercial and ecological applications.
diatoms
A primitive single-celled algae; also a common phytoplankton. Diatoms are unique for having hard, silica-based shells that stay behind when diatoms die to become "diatomaceous earth", which has many commercial applications because of its extremely fine pore structure. Main aquarium use: diatom filters.
discus
Beautiful South American cichlid with a distinctly round, flat shape and vibrant colors. They are a schooling fish, so keep them with at least 4 other discus. They are sensitive to water conditions.
display tank
A tank used to display and house fish under
normal circumstances, as opposed to quarantine tanks, which are usually kept mostly bare.
DIY
"Do It Yourself". A good way to tackle some aquarium projects if you have the skills or are willing to learn. Prime candidates for DIY approaches: sump construction and/or plumbing, cabinet building, co2 systems for planted tanks. Look online to see how others are doing it!
dosing pump / doser
A pump with a very slow drip ideal for adding trace elements, other supplements, or replacing evaporated water. The slow drip prevents shocks to the system that can occur with sudden changes, and allows minerals to dissolve more completely.
DSB / RDSB
See deep sand bed.
filter
A filter is anything that cleans water of debris, chemicals or organic waste. Most filters do this by forcing water through porous filter media. A filter may perform one or any combination of mechanical, biological, or chemical filtration; some run continuously; others purify tap water; some are specialized, like diatom filters or protein skimmers.
filter feeder
A filter feeder eats by filtering microorganisms out of the water that passes over and through its body. Filter feeders rely on proper water circulation to get all their nutrients. They include sessile invertebrates and some crustaceans. When filter feeding, you may need to turn off protein skimmers, UV sterilizers and other filtration devices.
filter media
The actual substance water is pushed through that cleans it; usually kept inside a filter, though it can include things like gravel in the tank. Filter media range widely in type and form. Since it does the actual filtration and is constantly accumulating the things it filters out of your water, good performance relies on replacing or cleaning it as needed. Common mechanical filter media include foam and floss, chemical filter media include activated carbons or ion-exchanging resins, while bacterial filter media is anything that provide good conditions for bacterial colonization, including live rock.
foam fractionation
The technical term for how a protein skimmer works.
fluidized bed
A fluidized bed occurs when fine grained particles, like chemical resin filter media or sand, are "suspended" in a chamber by a constant flow of water instead of settling on the ground (hence acting more like a liquid than a solid, or "fluidized"). This makes it an efficient filter, because it increases the surface area of the media and the amount of time it's in contact with water.
FOWLR
"Fish only with live rock". Describes a common configuration of saltwater tanks that use a structure built from live rock as the tank's focal point and primary filter. Success relies on a protein skimmer and being able to fit enough live rock to process the tank's bioload (with the exception of nano tanks, which may not need protein skimmers if diligently cleaned).
fragging
Another word for coral propagation. "Frags" or fragments are created by cutting or breaking a small piece from a healthy coral; the frag can then be situated in another tank. Captive coral fragging helps maintain the health of the world's natural reefs by decreasing the need for wild harvesting.
fry
Baby fish. Not to be confused with the other kind of fish fry.
fuge
Abbreviation for refugium.
gang valve
Commonly used with air pumps, the gang valve allows you to split a single airline output into several. Useful for powering multiple devices with your air pump, like airstones, skimmers, or action decorations.
gas exchange
Process by which oxygen dissolves into your water and unwanted gases escape; happens primarily at the water's surface and is aided by healthy circulation and surface agitation.
gH
"General" or "Total" hardness. Also called dgH (degrees of general hardness). A measure of all dissolved solids in water, including carbonate hardness (kH).
GPH
Gallons per hour. The measurement of flow rate mainly associated with pumps and filters, this describes how many gallons per hour a piece of equipment can move through it. Affected by head height, ball valves and sharp turns in plumbing.
halogen
Halogen lights are a type of incandescent light with a color spectrum range in the low red-yellows, bad for your aquarium because they will encourage massive algae growth, and also because they tend to run hot.
head height
The distance of a pump or filter from the highest point it must pump water to. Increasing head height will decrease GPH, and most pumps have a limit on how much head height they can overcome. (Pressurized pumps will be able to handle greater head heights.)
heater
An inexpensive device that heats your water as it flows past; generally has a built-in thermostat that you control. Essential to maintaining stable water temperature. Heaters are either fully submersible, immersible, or install inside or in-line with your filter.
herbivore
Organism that eats only vegetation. Aquatic herbivores usually
subsist on algae, aquatic plants, and seaweed.
hitchhiker
An organism that sneaks into your tank without your knowledge, usually either on live plants or live rock. Some hitchhikers are a nice bonus, like sponges or coralline algae, but most are pests, like aiptasia anemones, brittle stars, or some snails. If unsure of what a hitchhiker is, it's probably best to remove it.
HOB
"Hang on the Back", this generally refers to an external power filter.
hydrometer
Inexpensive device that measures the density of water. Used by aquarists to determine salinity, but lacks the accuracy of a refractometer.
immersible
Aquarium equipment described as immersible have parts that must remain above water but parts that can or must run underwater, so they hang over the side or back of your tank.
incandescent light
Energy inefficient with an inappropriate color aspectrum for aquarium use, incandescent lighting may be cheap, but boosts algae growth and can overheat water. Better suited to reptile tanks.
internal filter
Filters operated inside the fish tank. Corner filters, box filters or internal power filters are the main contenders (sponge and under gravel filters are usually just referred to as sponge or undergravel filters). Internal filters offer less surface area than external filters so are usually only adequate for smaller tanks.
invertebrates
Animals without backbones. These include crustaceans and mollusks, two families that comprise an astonishing array of striking wildlife, particularly in the marine environment. Of course, let's not leave out corals, sponges, anemones, starfish and other reef wonders.
iodine
A trace element found in natural sea water. It's removed by protein skimmers, so must be replenished regularly in skimmed tanks (and added when skimmers are off). Important to the growth of corals and crustaceans, particularly to coloration.
kalkwasser
German word that means chalk-water; kalkwasser is water with calcium hydroxide dissolved in it. Adding kalkwasser is the most common way to maintain calcium supersaturation and alkalinity in a marine tank where it is needed by growing corals, mollusks, and coralline algae. Adding with a reactor helps prevent calcium from precipitating, or turning into a solid, before it dissolves in your water. Also called limewater; the same results can be achieved using food grade pickling lime.
Kelvin (K)
The Kelvin spectrum measures the "color temperature" of different wavelengths of light. Low values (1000-3000K) describe the red and infrared wavelengths (you don't see these in aquarium lights, because they promote algae growth and don't approximate full spectrum daylight). 5000-7000K is a golden white, appropriate for plants and freshwater tanks. 10,000K is a strong white with a very slight bluish cast, while 20,000K has a stronger blue cast.
kH
kH is a measure of dissolved carbonates; the abbreviation comes from the German phrase for carbonate hardness.
krill
Krill are a type of zooplankton that resemble a microscopic shrimp. They are often used in fish food because of their natural pigment-boosting properties; a heavy diet of krill is the reason flamingos are pink.
laminar current
A current which flows in one direction only, such as the current in a river. Powerheads can be used to recreate this type of movement.
lift tube
A tube that supplies water movement to low-flow filter devices that may not use pumps, like sponge filters or under gravel filters. Lift tubes create large bubbles that move upwards, pulling water up behind them.
limewater
See kalkwasser.
livebearers
Fish that give birth to live young instead of laying eggs. Livebearers are often quite easy to breed. They are freshwater species, the most well-known being guppies, mollies, platys, and swordtails.
live rock
Rocks removed from a coral reef with organisms living in them kept intact. Live rock may be natural or seeded (inserted into a reef environment for a few years to accrue life). Live rock forms the structural foundation of a coral reef, and the bacteria & microorganisms living on them form the primary natural biological filter of the reef. It may be home to sponges, small coral polyps, worms, crustaceans, urchins - "hitchhikers" that may or may not be desired. Live rock must be cured before adding it to an aquarium. It is increasingly used as the biological filter in saltwater tanks or sumps because of its stability, easy maintenance, and natural status.
LCS
Term used in the hobbyist community for "local chain stores" such as Petco.
LFS
Term used in the hobbyist community for "local fish stores" not affiliated with chains.
LPS
Large polyp stony corals. These are the most difficult corals to propagate in captivity and are often difficult to keep.
lumen
Unit that measures visible light. Different light bulbs of the same wattage may have higher lumen output. Efficient lights have a high lumen to watt ratio.
macroalgae
Large algae with leaves or pods that resemble aquatic plants (as opposed to microalgae, which are mostly pests). Used in algal filters and as a live food for herbivores.
magnesium
Trace element present in the ocean important to the processing of calcium into a calcium carbonate skeleton by corals, crustaceans and mollusks. There's around 1275 ppm of magnesium in natural sea water.
mechanical filtration
The physical removal of solid particles from the water, usually by straining water through a porous filter media such as filter floss or foam. Protein skimmers perform mechanical filtration in saltwater using the surface tension of air bubbles. Keeps water clear and protects biological filters from becoming clogged with debris.
metal halide
Metal halide lights are a type of light bulb which burns very white and very bright. They require a special fixture and ballast. They are the closest artificial means of creating sunlight, and are used mainly for photosynthetic reef organisms and light-intensive planted tanks. They produce a beautiful shimmering water effect that characterizes reef tanks.
mulm
The proper term for that nasty gunk that accumulates in your substrate. Vacuum or siphon it out during weekly water changes!
nano
A recent trend in the aquarium hobby, "nano" means small and nano tanks usually refer to small saltwater or reef tanks around 20 gallons (the term is loosely applied up to 40 gallons, but 20 is a general consensus). Nano tanks must only stock small specimens that aren't in danger of outgrowing the tank or its filtration capacity. It is more challenging than keeping a larger saltwater tank since conditions shift faster in a smaller water volume, and saltwater organisms are very sensitive to changes in conditions. Nano tanks require frequent testing and water changes at least twice a week.
nitrification
The process by which ammonia is changed to nitrite, then nitrate, and finally nitrogen gas. See nitrogen cycle.
nitrate
NO3, a product in the nitrogen cycle. It is not toxic at low levels; freshwater fish can tolerate up to about 40ppm (lower is better) before experiencing discomfort. In a saltwater tank it should be kept as low as possible, under 10ppm. There are only a few ways to remove nitrates from water; the most effective is a simple water change, while plants and algae consume some. It is possible, but risky, to convert nitrates into harmless nitrogen (the true end product of the nitrogen cycle) with a deep sand bed.
nitrite
NO2, the second stage of the nitrogen cycle. It is toxic to most creatures, and should be at undetectable levels when a tank has cycled.
nitrogen cycle
The nitrogen cycle is the natural process of decomposition of organic waste. It's different on land and in water, but it happens everywhere and is the process of ammonia conversion into nitrogen gas by bacteria.
In an aquarium, decomposed fish poop creates ammonia - highly toxic. Nitrosomonas bacteria process the ammonia into nitrite, also toxic. Nitrobacter bacteria then convert nitrite into nitrate, which is much less harmful and can be controlled with water changes. This is as far as it gets in most tanks, though specialized systems like the deep sand bed break nitrates down to harmless nitrogen, which evaporates.
NSW
Natural sea water. Marine conditioners, supplements and salts must be carefully formulated to recreate the mineral and chemical makeup of natural sea water, which contains LOTS of important things.
omnivore
Organism that eats both plant and animal matter. The vast majority of aquarium fish are omnivorous.
ORP
Oxidation Reduction Potential. See Redox.
overflow box
If installing an external reservoir like a sump, you don't plumb directly into your aquarium, but into an "overflow box" situated in your tank. This prevents complete drainage of your tank in case of a power failure stops pumps from returning water, or a plumbing malfunction.
overstocking
A common problem in the hobbyist community, overstocking occurs when you have too many fish for the water volume of your tank, or when the fish you do have create a disproportionately large bioload. An overstocked tank is less likely to recover from changes in water condition and essentially keeps your fish living in a high risk state where a sudden increase to the bioload (overfeeding, death, new fish) could trigger spikes in toxic ammonia. Avoid overstocking - buy a bigger tank and be aware of tankbusters.
oxygen
All aquatic animals, including fish and invertebrates, need dissolved oxygen in their water to survive. So do the bacteria that perform the crucial task of biological filtration. Maintaining adequate levels of oxygen is extremely important and often overlooked; you can use test kits if you aren't sure, or add an air pump.
ozone / ozonizer
Ozone (O3) is an unstable and therefore highly reactive form of oxygen - a natural purifier because of its tendency to "oxidize" everything it touches. It quickly kills bacteria and other unwanted microorganisms, but must be administered properly, using an ozone reactor or ozonizer which controls ozone levels via an ORP monitor. Ozone causes rubber equipment parts in the water to become brittle and break. Too much ozone is harmful to any organism, including you.
peat
Peat moss is a type of dried moss that you can add to your filter. It softens water/lowers pH by adding acid and is useful in recreating a blackwater habitat.
pH
The "power of Hydrogen" - essentially how acidic or alkaline a solution is; one of the most important water conditions to monitor. Some fish need a very specific pH range to live, while many freshwater species can adapt to a range of values as long as they are stable. All organisms are sensitive to pH changes, which should be made very gradually if ever necessary. Regularly testing pH helps you catch problems early; pH naturally changes throughout the day, so readings should be done at the same time each time, and results recorded.
phosphate
A chemical compound found in tap water and sometimes present in small
amounts in fish food. Algae love phosphates, and phosphates in your
water mean you'll probably have to battle algae blooms. There are test kits to detect it and chemical filter media to remove it.
photosynthesis
Process by which plants and certain microorganisms use sunlight to create nutrients. A photosynthetic organism is one that gets most or all of its food this way.
phytoplankton
A photosynthetic microorganism that is a primary food source for marine filter feeders. Because they photosynthesize, phytoplankton are present in huge numbers during daylight hours, so feed invertebrates phytoplankton during the day and zooplankton at night.
plankton
Plankton is a microorganism that abounds in the ocean and is the world's
leading food source because of the huge amount and variety of life it
sustains. Phytoplankton are photosynthetic, like plants; zooplankton feed on other plankton. Any marine filter feeder's diet consists of both types.
power compact (PC) lighting
Power compacts are the same compact fluorescent lights that are poised to take over our daily illumination needs when incandescents are finally phased out. They use less energy than regular fluorescent lights but produce more lumens (more intensity, less wattage).
power filter
Technically any filter that's directly powered by electricity. External power filters hang on the back or side of a tank, while internal power filters are submerged inside. Capable of all three types of filtration (chemical, mechanical, biological).
powerhead
A small, submersible pump that creates one strong current. Multiple powerheads can be positioned for multi-directional water movement and if compatible, can be switched on and off strategically by a wavemaker. Used primarily to create currents (if general circulation is the goal, regular water pumps do that more evenly).
ppm
"Parts Per Million"; unit for measuring the presence of trace elements or chemicals in a solution.
prefilter
The pre-filter is any mechanical filter media placed so that water filters through it before it reaches other kinds of filter media. This means that all large particulate debris is stopped before it can affect the efficiency of more sensitive filters that are easily clogged. A prefilter should be rinsed often.
propagation
Propagation in the aquarium involves sessile invertebrates or plants, both of which can be multiplied by trimming small bits off and then resituating those in a different tank. With corals, this is also called fragging
protein skimmer
A unique mechanical filter which uses the surface tension of air bubbles in a tall column of water to attract dissolved organics; these organics - called "skimmate" - are then deposited in a collection cup which must be emptied every couple of days. The taller the skimmer's column, the more effective, and they only work well in salt water. They're powered by an air or water pump.
quarantine tank
Also called a Hospital Tank, a quarantine tank is a smaller secondary tank, cycled and equipped with a sponge filter, a heater, and some decorations or pvc piping. Its purpose is to house fish if they get ill, preventing spread of disease to the rest of the main tank, and eliminating needless medication. It is also used to house new fish for about 8 weeks before adding them to the display tank; new fish often carry disease that could take weeks to manifest. This also helps ease your fish through the stress of transition.
redox
Redox, or oxidation reduction potential (ORP), is a measure of how easy it is for chemical reactions to take place. This is an indirect indicator of water conditions and biological processes. It's usually only measured when an ozonizer is used, when an ORP monitor can detect and shut off ozone if levels get high.
refractometer
Device that measures the salinity of water by determining the concentration of one substance dissolved in another. More expensive than a hydrometer, but also more accurate.
refugium
A separate tank or container from your main aquarium that is plumbed to use the same water. Can be external to your aquarium or an internal separated compartment. Often combined with a sump filter. Used to house fragile or wounded organisms, raise fry, or culture live food & macroalgae.
reverse osmosis
A process for filtering impurities from tap water before drinking or aquarium use. Most commonly used in saltwater tanks where organisms are more sensitive to water conditions; R/O water can also be used if soft water is needed. R/O systems are pricey, filter slowly, and create several gallons of hard water as "waste" for every gallon of filtered water. Waste water can be used in gardens or in hard water tanks. RO/DI systems combine reverse osmosis with deionization, which is even more effective.
rift lake
A lake with unique water conditions due to its position above a geologic "rift" formed when tectonic plates meet, e.g. the San Andreas Fault. (Geology review: huge, slowly moving tectonic plates form the surface of the earth's crust.) African cichlids come from rift lakes on that continent.
R/O, RO, RO/DI
See reverse osmosis.
reactor
Device used to add a substance to aquarium water when control is important. Calciumkalkwasser, and carbon dioxide are often added with reactors, because they must be added very slowly to dissolve properly.
salinity
The measure of dissolved salts in water. Measured by hydrometers and more accurately by refractometers. The temperature of water matters to
accurate measurement, so good devices adjust for this.
schooling
Schooling fish are those which naturally travel in groups of the same species. Many aquarium fish are schooling fish, including tetras, barbs, and discus. Schooling fish will display vibrant behavior and cool patterns of movement if you keep them in groups of 5 or more; if kept alone or in too small of a group, they could show aggressiveness, shyness or other abnormal behaviors.
sessile invertebrates
Sessile invertebrates anchor themselves down for life - corals, sponges and anemones are popular examples. Sessile invertebrates may have a mobile stage early in development.
slime coat
Most fishes have a layer of "slime" over their delicate scales. The protective slime coat is a fish's first defense against disease causing agents (bacteria, fungus, parasites) and stress, and a healthy slime coat means a fish can recover from traumas quickly. Stress relief products boost your fish's slime coat; avoid handling fish directly so as not to disturb it.
specimen tank
Fish tank that houses only one species of organism. A better way to showcase a particularly large or aggressive fish.
spirulina
Thought of commonly as blue-green algae, spirulina is actually cultured
from a strain of cyanobacteria. Though it has recently become a popular
health food item, it's been cultured for human consumption for centuries.
It contains unusually high levels of proteins, fatty acids and vitamins, and is also a common ingredient in fish food.
sponge filter
A simple filter consisting of a large foam block connected to a lift tube, usually powered by an air pump. Performs mechanical and biological filtration. Its low flow rate makes it ideal for fry tanks or quarantine tanks. Bacteria can quickly colonize a sponge's porous surface, which also collects debris.
SPS
Soft polyp stony corals. These are the most prevalent corals on the reef and unlike LPS corals, are fairly easy to keep in an aquarium with today's advances in technology. Can also be safely fragged and traded, reducing strain on natural coral reefs.
silicate
Sometimes present in tapwater or certain substrates, silicates are nutrients that can create an algae problem, particularly brown algae. They can be tested for and removed with chemical resin media.
stress
Fish stress is similar to human stress, but more immediately dangerous. Caused when fish are handled, moved, transported in small plastic bags, and added to new tanks...can also be caused by frequent movement around the tank, aggressive tankmates, and not having safe places to hide (one reason decorations are important). Stress makes your fish susceptible to disease and death, so help him through tough times with stress relief products, and by providing hiding places, a calm environment, or using tank dividers to keep hostile fish away.
strontium
A trace element found in sea water that corals, clams, crustaceans and other skeleton-forming organisms need to process calcium correctly.
submersible
Describes a piece of equipment that can be safely used completely underwater. If you have a saltwater tank, make sure it is saltwater safe or the salt will corrode any metal parts.
substrate
The material used to line the bottom of your aquarium, most commonly sand or gravel. Substrate choice can affect water chemistry and well-being of tank inhabitants; some choices are better suited to saltwater or freshwater habitats.
sump
A container or tank used as an external reservoir, usually kept under the aquarium (plumbed into an overflow box; it requires a pump to move water back to your tank). Most useful for increasing the water volume in your system (adding stability), sumps often have multiple chambers that can be used to house equipment, wet/dry filter, or keep a refugium.
surface area
A measure of how much "surface" a substance has - a perfect flat surface has much less surface area than a surface with bumps, ridges or fine grains because of all those extra sides and crevices. Important to consider when choosing tank design (wider tanks have more surface area and therefore better gas exchange) and filter media.
surge
Water movement that flows in one direction and then in the opposite direction, usually with less force. Created by natural current patterns in the ocean; can be recreated in an aquarium with powerheads.
symbiosis
Symbiotic organisms rely on each other to survive. A striking example is that of zooaxenthellae algae, which live on certain corals and other reef organisms; the algae produces nutrients during photosynthesis that the larger organisms need for crucial stages in growth and development. In return, the zooaxenthellae are protected and sheltered.
tankbuster
Slang term for juvenile fish sold in the aquarium industry that will naturally outgrow most hobbyists' tanks. Sharks and catfish are the most likely culprits - many grow to be over a foot long and need lots of space. Do your research - buying a tankbuster leads to stunted, unhappy fish, and difficulty finding someone to adopt it when it inevitably outgrows you.
test kit
Test kits for aquarium use measure the presence of trace elements and chemical compounds in your water. They are often the only way to monitor important water conditions. They come with powder or liquid reagents, or use dip-strips (usually less accurate).
trace elements
Elements present in small amounts in natural fresh or marine water. Organisms get some of these minerals from food, but most of it must come from the water constantly passing through their systems. These must be continually added to your water with additives, supplements, or salts, as they are depleted by filtration and consumption by organisms.
trickle filter
Another term for wet/dry filtration, describing the process wherein water is slowly trickled over mechanical or biological media. This trickle exposes water to air, adding oxygen. Because nitrifying bacteria need oxygen, this is a superior form of biological filtration. The water may drip from a spray bar or drip plate.
UGF
See under gravel filter.
ultraviolet sterilizer
A lamp that emits UV light only. UV light kills unwanted bacteria and microorganisms. A tempered glass or quartz shield must be used to diffuse harmful wavelengths.
under gravel filter
One of the oldest filter designs in use. It's basically a perforated plate installed under the gravel of your aquarium. Water is pulled through the gravel, under the plate, and up through lift tubes - so the filtration is done by the gravel itself. While UGFs still have diehard fans, they lose efficiency quickly, are difficult to keep clean, and are detrimental to plants.
venturi
A specialized valve which creates a jet of bubbles by drawing air into fast moving water. Seen commonly in protein skimmers and aerating water pumps or powerheads.
water clarifier
A chemical additive that clears up cloudy water created by bacterial blooms or particulate debris. They should not be used as a primary method of keeping your water clear - this should be handled by using appropriate filters, cleaning regularly and doing regular water changes.
wave maker
Device similar to a power strip; when powerheads are plugged in, the wave maker switches them on and off at timed or random intervals to simulate natural ocean waves. Not all powerheads can hold up to a varying electric current, so check with the manufacturer if uncertain.
wet/dry filter
A form of filtration which adds oxygen to water as it trickles through media; because oxygen is needed by nitrifying bacteria, the wet/dry filter provides superior biological filtration. Can be found in sumps, some canister filters or rotating "paddle wheel" power filters such as the Bio-wheel.
zeolite
A natural, micro-porous ore used as a chemical filter media and also widely used in commercial water purification. Not effective in saltwater.
zooplankton
Microscopic organism that feeds on other plankton and microorganisms. Zooplankton increase in numbers on the reef at night, so if feeding your organisms phytoplankton and zooplankton separately, feed them phytoplankton during the day and zooplankton at night.
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