The little bitty snails in my pond eat the algea growing on my waterfall and decorative thingies. There never seems to be an overabundance, so I suppose the koi eat the extras. I also like them in my tadpole tubs.
I bought a blue and a yellow Mystery Snail at PetSmart (about 1½" round), to keep in my betta tanks indoors. MISTAKE! What nasty, filthy, heathen smuckers! They do nothing but eat and poop, the water fouls quickly and had to be changed every other day - and stunk!
I tossed them in the pond, where their poop will help feed my bog filter plants, and the snails are happily eating away on the sides of the liner... guess there's plenty of algae growing on it.