If you are looking for a snail that eats LOADS of algae but won't touch plants, get yourself some nerites. It's true they are a saltwater snail, but they adapt quit readily to fresh water. The best thing about them is THEY WILL NOT BREED in fresh water. Oh happy day!
I put 6 olive nerites in my betta 5 gal hex. The lighting I have on it, and lack of plants, algae was going nuts. These guys cleaned ALL of it in a matter of days. They have been living very well in the tank now for about 2 months. The guy I get them from has already acclimated them to freshwater. So all I had to do was a regular acclimation for new snails.
