I've kept freshwater fish for a few years now, but I've never included any sort of invertebrates in any of the tanks. I thought it might be fun to add a mystery or apple snail to my 20g, but I wasn't sure what to feed the snail. Do they just live off of the algae on the sides of the tank? Do they eat the normal flakes/proteins I feed the fish? or do I need to get other food for the snail?