Here is my two cents. UV has been used in Europe forever. All public aquaria use UV. Some ozone too but all use UV. I myself have been in this hobby since 1973 so a couple of years. Back in the day UV units made specifically for the hobby where all but useless. These days you can get the right sized unit for your setup without problem. Make sure you’re putting the most powerful unit your tank volume can handle at the slowest recommended flow rate without over heating your water. So here the thing and without getting into any arguments or being sent off to BRS experimental shop to let you know at a thousand miles per hour just what works because they “test it on their tanks so you don’t have to test it on yours”. UV kills everything that passes through it. It stops cripto from multiplying. Faster flow through it (if powerful enough) will wipe the floor with velvet. Someone already said in this thread that “if you have a full reef tank UV is useless”. I’d say the complete opposite is true. It’s almost essential. No matter how diligent your quarantine: one single drop of water even from the outside of a bag from your LFS can send ANYTHING into your display. You’re only friend will be UV. Just ask
AngelicaReef. Or if you need something disease specific from someone not looking to sell you something have a look what the
Ozzies do. Those guys who live next to the biggest natural reef on the planet. Happy Reefing.