If you really want to feed snails well, look up Snail jello. First class snail food, you make it yourself.
They are scavengers, so they eat almost anything they find, including dead shrimp, dead snails, dead fish, dead plants, whatever you feed fishes that ends up on the bottom. Though they do eat algae, they are actually omnivores that appreciate meaty meals now and then, as well as plants. Shrimp pellets will be appreciated.. they're not FOR shrimp, rather, they're made from shrimp, for fish.