I've seen my ghost shrimp prying freshly hatched pond snails off the glass and eating them. Also seen them working on larger shells, but don't know if they killed them, or if they were already dead. They don't seem to bother the ramshorns as much. My tank used to have a lot of pond snails and only a few ramshorns, after adding the shrimp, the ponds are dwindling, and the ramshorns taking over. 4 days seems pretty quick to wipe out a population though.