The stuff that I posted is called Poly-Fil. It's 100% polyester fibers. Don't use the kind that's marked as flame retardant or mold resistant ... it has chemicals.
A lot of dollar store sponges are biodegradable ... sounds like that's what happened to you. The carwash sponges tend to be ok, but what you want to look for in a foam sponge is one that water runs through instead of soaking like towel or dishwashing sponge would - and make sure it has no chemicals, is not biodegradable, etc.
Hi, I was reading your answer to the post "filter issues". You said to get this, but I don't know what it is. Is it cotton for stuffing dolls etc. with? I have a 2.5 gallon hexagon tank and a tom filter. I would rather use the cheaper stuffing in place of the sponge. I did try cutting som sponge I got at a dollar store, even though it specifically said on the label not for aquarium use, after 2 or 3 weeks, the sponge in the filter seemed to break up, making a real mess in Jewel's tank.