I was just reading about montmorillonite clay usage in tropical tanks and outdoor ponds. It is being said to be extremely beneficial and reason why Japan Koi breeders have such success (their natural ponds have clay mud bottoms). Among many other properties it should help in healing process of fish (and shrimp or snails).
It is being sold under many different names (and prices), but if you can get a Koi clay powder, IMHO it's worth a shot and not that expensive (and small bag will last you ages). Products for tropical fish or shrimp are sold in much smaller quantities and cost much more, yet the substance is the very same..