Ramshorn snails eat algae and won't eat the healthy parts of your plants, but will eat the decaying leaves/stems. Malaysian trumpet snails are safe as well. I don't know for certain if they eat algae, but they're good at sifting through substrate which prevents anaerobic pockets from building up (bad for your fish/inverts!). Apple snails are more colorful than the 2 aforementioned, but get more than twice as big as the other snails, about the size of a golfball. They're safe to have around plants and most likely pick at algae, although they need food in the form of sinking wafer/pellets, blanched veggies like zucchini, green beans, etc. in order for them to stay healthy and have pretty shells.