Outdoor goldfish are exactly the same as indoor goldfish, only they grow much bigger. A typical goldfish can grow to 12 inches or more, if its environment is right. The bigger the pond, the bigger the fish and it’s all related to the amount of oxygen the goldfish has available (surface oxygen).
If you want to have some outdoor goldfish to entertain your guest, or simply want a fish to keep your backyard pond occupied with life, then you need to make the pond habitable for them:
• If you have a winter freeze in your area, you need to provide a pond that’s at least 4 feet deep. If the outdoor goldfish in your pond get frozen, they’ll die. They will need at least a half foot of unfrozen water to survive the winter, since they just lay on bottom during a freeze up at pretty much hibernate.
• Goldfish aren’t hard for a predator, particularly flying predatory birds to spot and swoop down on, since goldfish prefer to stay near the oxygen rich surface of the pond. Depending on the circumstances you may want to attempt to fence the pond in, to keep your fish numbers high. Some Asian outdoor goldfish breeders train dogs to bark at the birds, which can be effective also.
• You can either put a pump driven filtration system in, or research way to create a natural filtration system in your pond to keep bacteria levels down and keep the water from clouding up. Don’t think the water will naturally keep itself clean, because the goldfish will reproduce and grow much larger than they would indoors making clouding just as much of a problem.
It’s hard to over-feed outdoor goldfish, since they have lots of oxygen to grow and digest their food. A pond with lots of bugs, frogs and other small insect and invertebrate life will be more than enough food to support the goldfish. However, this won’t always be the case and the number of fish that thrive in your pond will be a good indicator of whether they’re eating enough. To save yourself the worry, feed them outdoor fish food appropriate for goldfish.