I use them on both fish and reef tank. This topic gets i do/i don't answer. I’m not going to say you have to/need one, but remember anything you bring and put in your tank may not show signs of infections but that don't mean its not a host to parasites and bacteria and other "fun stuff" to corrupt your tank. Here is something think about, if your sick in a hospital, why do they put uv sterilizers in the hall ways? Sick people are in the room. People will say i never had one, never need one as long as i have a hospital tank and that’s fine, others will say money spent on something better. Uv kills free floating parasites and single cell algae and a couple of other things. For me its a piece of equipment I would just rather have, just incase. Who is to say for sure it don't do anything for any reason? Some one will. As for how well they work, in the time that i have been doing this hobby, starting out with fish only, I never had an out break and my algae is none i do turn it off once in a while and turn my mirror so the sun reflex a little into tank for natural algae to grow for my LMB and tang and snails, but have never had a problem of an algae bloom. Its a piece I think should be gotten after the other big important equipment is gotten.