IS there a downside to using a UV sterilizer every day?
In addition to the ones Musho mentioned (electricity cost isn't the only one, you'll have to buy new bulbs rather frequently, too. I think a bulb only puts out sufficient levels of radiation for around 6 months), I think that there is a psychological drawback, too. That is, that one will feel like "I don't have to do the weekly maintenance, I've got the super death UV thingy that will keep my water pure" when the weekly maintenance is still completely necessary. In fact, more maintenance is necessary, since cleaning the sleeve of the bulb to make sure the light levels stay high enough is an additional necessary step.
If you are using a UV to combat algae, for example, sure, you won't have algae itself floating around, but you are going to still have lots of undesirables in the water. Firstly, you will still have whatever was causing algae in the first place. Too many nitrates is a good suspect here. While nitrates aren't as deadly as ammonia or nitrite, it is still a toxin to the fish. Secondly, the UV does kill the algae, but what happens the the dead cells of the algae? They decompose and re-release their chemicals back into the water. The decomposition process takes oxygen, and the nitrates the algae was using to grow gets released back into the water. Maybe as nitrate, maybe as some other nitrogenous chemical like ammonia. In whatever form it gets released, it goes back into the water column. Net result is no alive algae, but you still have the same crud in the water in the first place.
This is again, why I emphasize addressing the root cause of the problems in the first place. I see UV as a band-aid, a patch to help stop the bleeding and prevent things from getting worse. But, it is not a permanent fix. The root issue should be addressed, and almost always, when the root issue is addressed, the UV band-aid becomes completely unnecessary.
