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Fish Tank Supplies > Aquarium Glossary

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!)

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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.

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.

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.

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.

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.

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.

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.

Describes an organism or process that consumes oxygen to function.

"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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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).

See water clarifier.

community tank
Fish tank housing multiple species of organisms that can live together peacefully.

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 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.

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.

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.

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.

"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.

See deep sand bed.

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.

"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).

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.

Baby fish. Not to be confused with the other kind of fish fry.

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.

"General" or "Total" hardness. Also called dgH (degrees of general hardness). A measure of all dissolved solids in water, including carbonate hardness (kH).

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 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.)

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.

Organism that eats only vegetation. Aquatic herbivores usually subsist on algae, aquatic plants, and seaweed.

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.

"Hang on the Back", this generally refers to an external power filter.

Inexpensive device that measures the density of water. Used by aquarists to determine salinity, but lacks the accuracy of a refractometer.

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.

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 ou

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