It depends on your tank size and bioload. Of course, bigger is better. Often the size is regulated by the place you want to install it, such as under your stand. The tank of the month has a 30 gal on a 90 gal tank. I have a 10 gal on my 20 gal, but will soon have a 20 sump/fuge shared by the 20 and a 55.
