sorry to hear about your fish contamination from snails is a huge issue they are the greyhound bus for paracites usually they are an intermediate carrier for paracites but what would cause problems. paracites get their name because they are paracitic on their host the first place they go is the gills it is the most efficent way for them to get some food hence your breathing problems i might dip your fish or at least thats what Dr. David Huffman at TX state says might help (paricitologist) same thing is going on with the fountan darter a fish in the sm river someone threw a asian snail in the river with centrosestus formusanus a paracite using the snail as a host that attached its self to the gills of the fish and since darters lack a swim bladder they drown becuase they cannot gulp the air from the surface. get your snails out of your tank they are still hosts that is if it is a parasitic problem get a UV sterilizer i would think clean the system out at least thats what i would do if you want to learn more about paracites ask you wont eat pork anymore...