Cameron, The Hydro sponge filter box says HS II is for a 20 gallon tank. I happened to have a Hydro sponge IV in one of the 20L breeding tanks. The tank is heavily planted and the residents (A. borellii) seem to be really happy, multiplying like crazy. Another heavily planted 20L has a Tetra's brilliant filter sized for a 20 gallon tank. Each filters is driven by a small size air pump. I really don't see any difference between the two tanks. Hope this helps, Tomoko ------------------------------------------------------------------------- This is the apistogramma mailing list, apisto@listbox.com. For instructions on how to subscribe or unsubscribe or get help, email apisto-request@listbox.com.