Ultraviolet units have two main functions in fishkeeping, with the most common being algae control in ponds or aquaria located in sunny positions and stocked with messy fishes. These ‘clarifiers’ are designed with a void space that exposes the target organism (algae) to the right amount of UVC rays to destroy them as they pass through the killing chamber. A steriliser has a smaller void space for killing more complex life-forms, such as protozoan parasites, as well as being lethal to the same types of algae which a clarifier is designed to kill. As they reside on solid surfaces, filter bacteria are generally unharmed, but it may be wise to delay running a steriliser until your tank is cycled.
UV sterilisers are a common feature of commercial systems and give chemical-free control of parasites such as white spot, as well as some harmful bacteria. Long used by marine hobbyists, a UV gives protection to systems that contain invertebrates such as shrimp and corals which are harmed by medications which may be needed to deal with disease outbreaks in fish tankmates. Disease control is the primary function of a UV and for systems where copper-based treatments are off the table, these devices can be a literal lifesaver. We’d always recommend a quarantine tank for treating new arrivals prior to adding them to these kinds of systems but this isn’t a viable option for some keepers and is all too easily short-circuited by just one livestock addition.