Is this tank planted? Apple snails DEVOUR plants
Most apple snails are voracious plant eaters (herbivores or macrophytophagous) that eat a wide range of vegetation. Held in captivity, they do well on common vegetables in combination with fish food.
Unfortunately, many species have a great appetite for aquatic vegetation and algae are not their preferred food. In such cases these scavengers can reduce the aquatic vegetation very quickly. They can ruin a beautiful aquarium within days.
Sourced that from applesnail.net