Have you tried to grow them in a mixture of peat moss and sand? This is what I grow pitcher plants in and they are submerged in water and don't get mold. I use the same mixture to root cuttings in.

No, I hadn't tried that. I actually recently singled out my favorite and set it up under the grow light in some flooded, very fine soil, and it seems to be doing better on the mold front. I'd still like to be sure though, so if anyone knows anything I might add to the water to keep it healthy and fungus-free? Some sweet potatoes I tried to root in water in the same spot got pretty gross so I know spores are about.

It may be too late but hydrogen peroxide is probably your best bet to soak the corms and add to the water. I ran across a thread from Asian Vegetables that might be helpful.

I've never tried this but I suspect you might not have much success indoors.

Here is a link that might be useful: growing water chestnuts