I feed mine algae wafers every other day and occasional flake fish food maybe once or twice a week. they also like vegetables, such as romaine, cucumber and green beans. check out applesnail.net. Lots of good info there. they also appreciate calcium in the water for their shells. If you aren't using coral gravel or something like that, you can actually feed them human calcium pills, such as Caltrate, preferably without Vit D. Most snails will just eat it up!
55 gal with 3 fancy goldfish. 2 goldies in QT, 23 flatback hex tank with betta, 3 ADF's, 2 BN's and nerite snails, 60 gal community tank: 4 angels, rainbow shark, raphael cat, apple snails,etc.