People have kept goldfish in ponds for centuries, if not longer. However, many species of fish cannot safely share a pond with goldfish for a number of reasons, including incompatibility of requirements, varying immunity to diseases and the risk of predation. Only a handful of species of fish can safely share a pond with a goldfish.
Not all goldfish belong in the same pond. Goldfish broadly fit into two categories: common and fancy. Common goldfish resemble wild goldfish, with more coloration. Fancy goldfish have been bred to have longer fins and unusual body shapes. When kept together, common goldfish tend to out-compete fancy goldfish for food. Fancy goldfish swim slower due to their ornate finnage. Additionally, if they share a pond, fancy and common goldfish can cross-breed, since they still belong to the same species. This tends to produce fish that are a regression toward the wildtype, undoing the generations of breeding it took to produce the fancy goldfish.
Koi belong to a different species than goldfish, but are a related species of carp. Both fish have similar requirements, though koi can grow larger. Due to this larger size, a pond with koi will need to be larger than a goldfish-only pond. Also due to their larger size, you need to keep koi only with larger, fully grown goldfish, since koi can get big enough to eat smaller goldfish.
Minnows and Others
Some minnows can also share a pond with goldfish. White cloud mountain minnows, rosey red minnows and some danios can comfortably share a goldfish pond. When selecting a minnow, make sure it comes from subtropical or temperate ares, since tropical species cannot survive outdoors in most parts of the United States and Europe. Keep in mind that most minnows stay small, and a healthy goldfish in an outdoor pond can grow larger than a foot in length. This means that the goldfish may snack on your minnows.
What to Avoid
Goldfish should never share a pond with tropical fish. Tropical fish cannot survive in most outdoor ponds over the winter. Additionally, goldfish tend to grow larger and faster than most common tropical aquarium fish, turning tropical fish from tank mates to prey. Additionally, goldfish and tropical fish come from different areas and climates. This causes goldfish and tropical fish to have resistances to different diseases, allowing them to be carriers for diseases that can kill each other. Do not keep goldfish in a pond with tropical fish.
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