We’ve got a fish problem: our pond is the main part of an old moat, one arm of which is in a neighbour’s garden. The neighbours keep fish, and have put a fine wire mesh across the narrow connecting part at the boundary to keep them on their side. Unfortunately, very young fish got through it and we are now infested with small grey carp of some kind, including goldfish, which appear to be breeding. The amount of insect life in the water seems to have dropped sharply, and suspicion falls on the fish.
There are also three big grass carp which arrived in an earlier escape three years ago, but we rather welcomed them. They are entirely vegetarian and seem to be keeping down invasive weeds including duckweed.