Algae, so you don't really need to feed them as they "feed" themselves.
Slowly.
I have a large apple snail, about a baseball size. I feed it only with Romain lettuce. It survives well.
leaves!
leaves
feed it
Nope.
there in water
Twice a day
Damp areas, with plants to feed on.
Throw it away or feed it to a turtle.
snail food
leaves