Just wanted to add my phosban reactor - reactor in here. Pretty simple and same idea as the rest. The reactor is too restrictive for a tank my size (165 gallon) but as inkslinger said it would probably be good for 55 or less. It's building up gas for me at my current feed rate but I think it would run fine for a smaller tank. I have mine fed by a Eheim 1250 right now. It would probably be fine if the input/output weren't so small but I need a bit more flow.