Goldfishes in ponds

"Goldfish are popular pond fish, since they are small, inexpensive, colourful, and very hardy. In a pond, they may even survive if brief periods of ice form on the surface, as long as there is enough oxygen remaining in the water and the pond does not freeze solid.

Common goldfish, London and Bristol shubunkins, Jikin, Wakin, comet and sometimes fantail can be kept in a pond all year round in temperate and subtropical climates. Moor, veiltail, oranda and lionhead are only safe in the summer.

Small to large ponds are fine though the depth should be at least 80 cm (30 in) to avoid freezing. During winter, goldfish will become sluggish, stop eating, and often stay on the bottom of the tank. This is completely normal; they will become active again in the spring. A filter is important to clear waste and keep the pond clean. Plants are essential as they act as part of the filtration system, as well as a food source for the fish. Plants are furthermore beneficial since they raise oxygen levels in the water.

Compatible fish include rudd, tench, orfe and koi, but the latter will require specialized care. Ramshorn snails are helpful by eating any algae that grows in the pond. It is of great importance to introduce fish that will consume excess goldfish eggs in the pond, such as orfe. Without some form of population control, goldfish ponds can easily become overstocked. Koi may also interbreed to produce a sterile new fish."
From Wikipedia.org