My goldfish are currently kept in an outside pond, and in the climate here, Melbourne, Australia, that’s fine. Of course, if you have severe winters with snow and ice, then it may well be different for you. Research online, or with your pet shop for local conditions and recommendations.
I’ve recently discovered that the cause of a high rate of mortality in goldfish can very be toxicity. This is caused by fecal matter building up in the acquarium or pond, especially if you don’t have a filter in a small area. The water becomes low in oxygen and the fish will die, almost overnight. A filter is good, but if you can’t have one, then at least one third of the water should be changed weekly.
When changing the water, don’t use it directly from the tap, as there are chemicals in most water supplies. Age the water for at least 24 hours in a bucket, or use rainwater from a tank. If there are mosquito larvae in the tankwater, don’t worry about it, as the fish will be delighted to have them!
Feed your fish regularly, but not necessarily every day, particularly if they are in an outside pond, with plants. If there are acquatic plants, the fish will browse on both them and the algae growing on the leaves and the pond sides. I’ve even seen a large goldfish grab a snail by the horn and pull it into the water, although whether it ate the snail or not, I don’t know. Perhaps it had to wait until the snail softened up.
Although I didn’t really know much about goldfish, I’ve been lucky, and in the past ten years, my pets have bred four times in their little pond in my garden. On each occasion, I’ve not known they had bred until cleaning the pond, and discovering small fry in there.
It’s a pleasant surprise to find young, as adult goldfish, of course, will eat any fry they catch. Some thick water weeds make good cover for the tiny fish, and fortunately, my pond plants grow very quickly, enabling some of the fry to survive.
The first time my goldfish bred, I had nine babies. Shortly after that, however, we had toxic algae bloom in the pond, and most of the fish died. There were two survivors.
After this happened, I finally purchased a filter for the pond, and the group of goldfish we had at the time were looking very fit and healthy.