Aquarium UV Sterilizers
Aquarium UV sterilizers use high intensity ultraviolet light to kill free floating microbes - bacteria, fungi, viruses, parasites and algae (only free-floating; they won't cure illnesses). Aquarium UV sterilizers lack risks associated with ozonizers, but aquarists are divided on whether to use them - a well maintained, appropriately stocked aquarium should be fine without one. But many do use them to improve water quality, and they clear up lots of persistent problems quite well.
Most bulbs need replacement every 6-8 months (never look directly into a UV bulb; it could damage your eyes). For more replacement UV bulbs, sleeves and parts see our UV sterilizer parts.