I need some help with this filter issue as well.
I have a 20 gallon, divided twice with 3 male bettas, so far everything is ok expect that current.
I've tried it with all three and none really appreciate it, seems to be too strong, and i cant find a way to slow it down.
Can you please show or explain the sponge idea or the nylon thing??? I've read about it but dont understand how to do it or if i need a different filter system.