How necessary are UV filters?  Are they do or die?  or just a gadget?

Looks like they are very important in systems housing lots of fish.

There are times I would definitely use a UV and other times I think they are a waste. It depends on the application. What are you doing?

I'm breeding clowns right now, hopefully a bunch of different fish soon.

If you are looking to sterilize water that is flowing between a multitude of aquariums on a central system, large commercial UV's are a good idea.

I'm fully inclined to agree with Mr. Pro. The fact is that in breeding setups, we the ratio of fish to water is such that when one fish gets sick, they all could. All the normal things that prevent disease, be it immune response, water quality, natural 'removal', you name it, is at a disadvantage when you cram that many fish in one space. Think about it this way. If you're in a gymnasium with one other person who has a cold, how likely are you to catch that cold. Now fill that same space with 200 other people. How much more likely are you, and everyone else, to catch that cold?

I would say that in a breeding setup, UV is not-optional in my opinion. The age old "ounce of prevention vs. pound of cure" argument definitely applies.