pond snails. i took out the only one i could find today. i had 4 in their yesterday, and today only one remained, so i took it out and put it back into my mbuna tank where it had layed eggs a couple days ago.
It depends a lot on the fish and the snail. I had 3 apple snails (1"-2") I kept in a tank with my GT that he never bothered, and I have heard that the shells on Malaysian Trumpet snails are too hard for most cichlids to eat.
I think that if the snail is small enough to fit in your Jack's mouth, it will probably be eaten.
the thing is that i never know how much my JD can fit in his mouth cuz he keeps it closed most of the time. he is at least 3 times bigger than the ones they sell at petco.