Some snails are very good indeed. Their eggs and tiny hatchlings provide food for fish. Unfortunately we have been encouraged to buy pond snails which are often completely unsuitable. Just like land snails, some pond snails are actually a pest and will seriously damage the planting in your pond. They breed easily and can grow disturbingly quickly in numbers. Those to avoid are the ones with spiral, pointed looking shells. Flat shelled snails are less of a problem and ramshorn snails are great because they eat algae. Unfortunately it is hard to avoid inadvertently introducing these common pests as so many aquatic plants will be bought with eggs already sheltering in their leave. They are equally easy to get rid of though. Float some tasty looking leaves in your pond at night. Lettuce will do. Then in the morning net the hapless munchers who are stuck to the leaves and chuck them away.