I've seen some lfs has sponge filters in their piranha tanks before so it's probably ok. they sell those once that are inside a plastic housing so you don't have to worry about your piranhas eat the sponge. IMO those don't look nice in your own tank. I wouldn't do it myself unless it's a hospital tank or a holding tank for short period of time or a grow out tank for fry.
ps. you should post this in the main piranha section, not media lounge section.