Originally Posted by Elliriyanna
10 gallon is the one I am considering converting.
From my experience, a 4x4x4" sponge filter with 15PPI can handle 3 bettas and a snail in a 10G divided tank. The PPI (Pores Per Inch) is an indication of how much surface the sponge has (in order to house the Beneficial Bacteria), the smaller the PPI, the more surface. I use 50 PPI sponge for the fry tank.
For more infos see this page (where I buy my Sponge filters from):