After having a load of mosquitoes in the water butt I put a few goldfish in there and they did fine for a couple of years, even tried to spawn regularly but the hot weather this summer did them in. They've always been fine with the regular water changes off the shed roof but there wasn't enough this year. I've upgraded to an ibc tank a couple of weeks ago and put a fountain pump in for a bit of oxygen. The water was going really cloudy though so I've knocked up a biological filter using the water pump in a plastic drum and some home made filter media. Cost so far for a 1000 litre pond is 25 qu