Another good one that I have found to work well is the Nerite Snail. They are smaller than apple / mystery snails. They come in many cool patterns, the most commonly seen of which is marketed as the Zebra Snail. Nerites od not breed in fresh water and eat green algae exclisively. So if you have no algae you'll want to give them some boiled kale or frozen, defosted spinach to eat.
I would recommend 1 nerite snail for every 5g.