I would never put apple snails with africans, their antennae are way to visible. Not only that, they are cruddy algae eaters. Assassins would do nothing for algae.
Your best bet would be a nerite. I would start with an olive nerite as they are the cheapest. These do not reproduce in freshwater, have very tough shells, not very visible faces or antennae, and are exceptional algae eaters.
Your best bet would be a nerite. I would start with an olive nerite as they are the cheapest. These do not reproduce in freshwater, have very tough shells, not very visible faces or antennae, and are exceptional algae eaters.