I used one in my 20 gallon tank and it works great. Like reefrunner said they are good for flow, mechanical and chemical filtration. The tidal flow thing can be a bit touchy but when it works its almost like having a wavemaker. You can even get the heater for it and keep the inside of the tank neater. The only problem I had with it was sand ruined the impeller and I had to replace it(the impeller).