I have golden mystery snails. They get the size of a golfball and help with algae and with excess food if you overfeed. They are $1 each or 6/$5 plus shipping. They lay eggs above the water line so you can remove if you dont want babies.
I also have malaysian trumpet snails. They help with churning gravel or sand. They pretty much come out only at night. THey eat excess food. Some call these very helpful, some call them a pest snail. They multiply rapidly if overfed and give birth to live young so difficult to control them from taking over, but it can be done. These are 10/$2 plus shipping and only get about an inch long.
Look up pictures and descriptions to see what you like. If it is for the look, then i reccommend mystery snails or nerite snails. Both have many different colors.
Good luck deciding.