I don't know of any major impact that snails will have on your pond. The snails may carry a parasite that could cause grubs in the fillets of your fish. They're tiny white grubs that get into the fillets of the fish. Although they're not what you want to see when you cut them open, from what i've read they're harmless to humans. I don't believe that these parasites or the snails will ruin the pond. Adding redear to the pond could help with the snail population, but if nothing else it'll add diversity to your pond which is a plus. Hopefully someone else can give a more definitive answer than I, but I just wanted to tell you what little bit I know about snails. Good Luck
