I don't know about the US, but in Europe you have to be carefull with sponges you buy in supermarkets and other home appliances stores, because lots of them come with chemicals embedded in the sponges.
It is qite frequent for newbies around these parts to simply destroy a tank on account of badly chosen sponges.
It is more expensive, I know, but be sure to by inert materials and not all home material, albeit seeming so, are inert.
M