There are a few species that can survive alongside goldfish, including Zebra Danios and White Cloud Mountain Minnows. These fish should never be added to a goldfish tank if the goldfish inside are larger than they are, since large goldfish may eat them. Goldfish should optimally be kept in a tank only with other goldfish.
Experts warn that the unique needs of the goldfish can't be adequately met with other types of fish in the same tank because goldfish really thrive best with other goldfish in low-temperature tanks. Never mix tropical fish with goldfish since tropical fish need a warmer tank and have different nutritional requirements to goldfish, which makes it impossible to feed both types of fish adequately. Goldfish are also fast eaters, often pushing slower, smaller fish out of the way in order to eat.
Goldfish are considered peaceful fish, while many types of tropical fish can be aggressive. Keeping goldfish and tropical fish in the same tank together can cause injury to the goldfish, including nipped fins and more serious bites.
Because goldfish are social creatures and enjoy company, it is a good idea to keep them together with other goldfish of similar size and shape. Keeping different species of goldfish in one tank is also not recommended.