I am not sure who said BB is in the water...maybe some, but my understanding has always been that it is on/in things like sand, live rock, filter sponges etc. I do not think that UV
eliminates much BB, which is what someone else had asked about.
I had one on an old reef tank and did not see much benefit. Really no difference after I decided to remove it. It sounds like they are less expensive now and I do not think it will hurt to buy one. Just not sure there is as much benefit as some will expect.