A few goldfish would sort out the problem. We have had goldfish in our pond since we put it in, 35 years ago, we never feed them, they are just part of the system within the pond. They do breed but don't get very big, and the visiting heron is welcome to a few if he can catch them. After a few years they revert to being their natural brown colour in any case. I suppose people who breed them for sale weed out the brown ones. We also have thriving colonies of frogs, newts, dragonflies (3 kinds) damselflies etc and the goldfish don't interfere with any of the other many species. The edges of the pond are just wild plants and recently we gained marsh orchids from somewhere, I really don't know how they got there.