They will eat anything that can't move quickly so will leave fry alone. I have pomacea bridgisii (apple) and neritina natalensis (zebra). The latter lay eggs under the water and both the apples and zebras leave them alone so you SHOULD be ok, never had fish eggs in thier tank though. I feed them flake and lettuce most of the time, but have put in a dead fish now and again and they are gone in the morning, and I mean completly gone.
Jon