UV sterilizer
Device that exposes water to ultraviolet rays to kill pathogenic bacteria, free-phase parasites and suspended unicellular algae.
Detailed explanation
The UV sterilizer uses an ultraviolet lamp (UV-C at 254nm) enclosed in a quartz tube. The water flows around the tube and the UV rays damage the DNA of the microorganisms, killing them or making them unable to reproduce.
UV is effective against: suspended pathogenic bacteria, parasites in the free-living phase (Ichthyophthirius trophozoites, Cryptocaryon dinospores), unicellular algae causing green water, algae spores.
UV does not kill parasites when they are on the fish's body (sessile phase) but intercepts them when they swim freely in search of a new host. For this reason it does not cure an ongoing infection but slows its spread.
The UV power needed depends on the lens. To eliminate unicellular algae, 15,000-25,000 uW*s/cm2 are enough. For bacteria you need 30,000-45,000. For parasites 90,000-100,000. Translated into practice: low flow through the sterilizer = longer contact time = greater effectiveness.
The sterilizer must be sized for the volume of the aquarium and connected to the filter delivery line or with a dedicated pump. The flow must be slow enough to allow adequate exposure.
The UV lamp degrades over time even if it continues to emit visible light. It must be replaced every 6-12 months to maintain effectiveness. The quartz tube must be cleaned periodically from limescale.
UV is not a substitute for quarantine. It's an additional layer of protection. In marine aquariums with many fish, continuously turned on UV significantly reduces parasitic problems.
UV also kills beneficial bacteria and copepods that pass through the stream. For this reason some aquarists turn it on only when there is a problem, not continuously.