Here is my take on this:
UV sterilizers
Ultraviolet (UV) sterilizers are also sold as a “cure” for Cryptocaryon. The problem is that most hobbyist-sized UV sterilizers do not have the power to make an effective kill on the relatively large Cryptocaryon parasite. Additionally, UV sterilizers are effective only on the tomite/theront stage, as this is the only point where the parasite is even present in the water column.
The question here is that tomites/theronts must leave the fish. Actually, some of them may get caught up in the fish’s mucus and stay attached until they become infective trophonts again. This means that UV sterilization will not eliminate active Cryptocaryon infections from a single aquarium. Where it does have benefit is in eliminating tomites as they pass through a filtration system from one discrete tank to another (like in a public aquarium or fisheries lab). Decades ago, diatom filters were touted as cures for ich and velvet. The same issue applies with them; there are adherent forms of these protozoans that can continue to infect the fish without ever having to leave the fish’s body. Even if they do, the “dwell time” factor means that some theronts will still be present in the water column to infect the fish
The argument can also be made that these units have no controlling effect on disease organisms that live on the fish directly and do not need to leave the fish during some part of its life cycle (some protozoans, trematodes and most pathogenic bacteria). The expected benefits from a properly operating UV sterilizer are simply a lowering of the free-floating bacterial and protozoan populations, as well as fairly good control (close to 100%) of these organisms when water flowing from one tank to another first passes through the UV sterilizer. Side-stream sterilization, where the irradiated water is returned to the same tank it was taken from is always less than 100% effective – too many of the target organisms are able to escape passing through the unit and are able to continue to reproduce. So; if one accepts this position, why then are UV sterilizers so prevalent? The primary reason is “they can’t hurt”. Using one on a system has no discernable drawbacks aside from cost. Secondarily, their use is firmly entrenched in both public and private aquarium industries. Finally, as mentioned, they do have specific uses for which there is some benefit. That said, do not expect these devices to control active diseases in your aquariums.
Never expose aquarium water treated with chloroquine to ultraviolet light, either through UV sterilization or natural sunlight. It is suspected that UV reacts with chloroquine to create compounds that either are toxic to the animals, or that reduce the effectiveness of the treatment – or both.
