Sponge filters are perfect for anabantids that need minimal water flow (if a filter is needed at all). So I guess the question is, how much water flow gets through the dividers?
An even easier solution is, no filter. Provided you have live plants in each section (and betta need plants
) this works very well. I have a 10g with no filter, no light, just a heater, with 9 Boraras brigittae and a Corydoras pygmaeus (one lonely fellow, all they had at the store, will add 5 more as soon as I find them). Full of plants. There's a photo under "Aquariums" below my name on the left, the 10g Sand tank.