Freshwater Snails

Do freshwater snails filter the water and are they considered beneficial in Koi ponds?

Freshwater snails are good for two things:

1. Eating Algea
2. Reproducing

and reproducing and reproducing and more reproducing. Usually adding snails to a pond is a bad idea, they can overtake a pond, clog mechanical filters, and be tough to get rid of. A good filter system is the most beneficial thing for a Koi pond.