Something you can do is treat your tank with a powder/solution called flubinol 15 (look up 'skinny disease'). It wont harm your fish in any way - it's basically a wormer so it could do some good. Anyway, from my own experience, the stuff is ab fine for any sick/healthy fish, but comes with one added benefit. It kills all snails. Not sure why, maybe they're simiar to internal worms in some wierd way. You'd hav to make sure that you took all the carcasses out though, otherwise it'l send the water parameters in your tank crazy.
Just a thought