Other than mold, I'd make sure to wash the sponges really well before putting them in there. Many are treated with anti fungal stuff, especially the ones that you get that are 'damp' in the package. The Ocello brand also is listed as specifically not to be used in aquariums because of the chemicals involved so though they are cheap, I would check to see if they are safe. Here's the chemicals that are used from a couple of fish forums:

The humectant (moisturizing agent), Magnesium chloride, found in both Scotch-Brite(TM) and O-Cel-O(TM) sponges and the anti-bacterial agent, Zinc pyrithione, are poisonous to fish. All packages provide warning: "Not for aquarium use."