"I've had this teeny tiny filter on a teeny tiny 2.5g tank in my office for about 6 months, and it is fantastic. It's virtually silent and so efficient - the water is absolutely sparkling clear all the time.
It also has a very adjustable output flow for such a small filter, which is good because I have a paradise fish who likes to swim in the current of the filter, but doesn't enjoy a tsunami effect in his itty bitty tank.
I've kept tanks of all sizes since I was a kid, and I've used all sorts of filters, and I've never found a small one to beat this little guy. It keeps my tiny tank as clean as my canister filter keeps my larger tank. I've been very, very impressed."