I love both of my Aquatop canisters. I have the 500uv on my 90 gallon (with a sponge filter and a bio-wheel 350 backing it up) and a 400UV on my 55 gallon (with an internal filter and a sponge filter backing it up). No problems with either, and I've been running the 500UV for a few months now. I'm strongly considering picking up another 400UV for my 30 gallon as well.
