Canned spinach is a favorite of apple snails. It gives the what they need for their shell and makes them grow fast. Keep in mind that as the snails get bigger they need more and more food. Get a can opener that breaks the seal on the lid, that way you can use a rubber band and keep the can in your fridge using the original lid to keep the spinach fresh.
I had my apple snails lay eggs a few times, but they were too close to the light so they dried out. (They were only going to be fish food anyway). So, if they are close to the light you might want to move them.
Jeez I was going to suggest some copper based solution to kill them all. I didn't know many people actually raised them lol. Usually snails are a pest, more than anything
In my experience apple snails weren't a pest, as they aren't hermaphrodites (so you need a male and female to breed), and lay their clutches of eggs above the waterline, where they are easily plucked off if they are unwanted.