Fish that can coexist with goldfish include white cloud minnows, rosy red minnows, rosy barbs, charming weather loaches, zebra danios, tetras, guppies, wimples and other goldfish. Goldfish may eat smaller fish, so potential companions should be of comparable size to their goldfish tank mates.
Goldfish are not territorial and should not be placed in aquariums with aggressive fish. Goldfish should also not be included in tanks along with tropical fish as their environmental requirements are significantly different. Goldfish are freshwater fish that prefer a water temperature that never exceeds 64 degrees Fahrenheit. Higher temperature water causes goldfish to asphyxiate as they are unable to breathe in less-oxygenated water.
Goldfish are also omnivorous and require a different diet than carnivorous tropical fish. The best tank mates for a goldfish are other goldfish. However, it is important to mix goldfish that are of the same type. For example, fancy goldfish should be kept with other fancy goldfish because they are relatively slow moving. If fancy goldfish are mixed with faster swimming comet goldfish, fancies may fall prey to malnutrition as their quicker counterparts eat all of the food.
It is also important to consider tank size when including multiple goldfish in a single aquarium. An adequately sized aquarium for a single goldfish is 5 to 10 gallons. The tank volume should be increased by 5 gallons for each additional fish.