If you have only snails in your aquarium, you won't need to feed them much more than snails living with fish. Give only as much food as they can eat before the food goes bad. Store-bought food will last longer than produce. Start small when feeding produce and gauge how quickly your snails chow down on the fresh goodies. A tank with only a few snails will take a while to get through lots of produce, especially when plants and algae are available. If you've got several snails in the tank, anchor your produce in various spots around the tank so the snails can access it at several locations.