I read somewhere that Snails can die from too much Algae (Overeating? - Maybe their shells do not grow fast enough to keep up with the growth of the snail itself? - that is my best guess at that Theory).

Your mag is very low and I read somewhere that that can be the problem.

Lastly you could have something that came in the Live Rock that eats them (Crab or Large Worm).

Check at Night when these hitchhikers come out to see if you notice any that should not be there that you did not buy.

I have never had the wrasses that you do but some wrasses do eat snails and shrimp.