My
MTS (Malaysian trumpet snails) are great for preventing toxic gas pockets from forming in my sand substrate, and they are experts at eating algae and uneaten fish food. They are ideal for planted tanks with sand substrate. MTS do multiply like mad, so you need to make sure not to overfeed them. I just wish that MTS had more vibrant color varieties.
I have one black mystery snail and one golden mystery snail. These snails are not very useful for moving around sand substrate, but they have beautiful colors.