Although many sponge filter will claim they are usable to an X size aquarium the actuall GPH
is ALL on the air pump.
The amount of space for BB is what is limited for the most part by the sponge itself.
If you want more flow get a stronger air pump OR even use a powerhead on top of lift tube from sponge.
I have run my 75 g packed with swordtails(young adults(100+)) completely with only 4 sponges.
It probably could have been only 2?
Water changes and rinsing the sponge is key IMO
since they also polish the water better then most common filters.
Enjoy your sponge filters,I sure do!