So I confess, I have never used a sponge filter. However, my bettas continue to have frayed fins and I change their water every 2 days. Ammonia 0, nitrite 0, and nitrates 10. KH 5/89.5, GH 9/161.1, and PH 7.8. I added some IAL and driftwood for PH. Only thing I can figure is the Whisper hob is pulling their fins in. Sorry for long post. I have ordered sponge filters to see if they help. Do they contain charcoal? How long does a sponge last on them? Do you clean sponges just like you do filter media bi-weekly in tank water? Anything else I need to know? If this doesn’t work then I give up!