When keeping different types of goldfish together in an aquarium, there are several things that should be avoided. Goldfish with large size discrepancies should not be mixed together, because the smaller goldfish could have harder time trying to get its fair share of the food. Another thing to avoid is combining slow swimmers with fast swimmers. For example, fast and agile goldfish types such as the comet and common goldfish should not be mixed with slow movers like the ranchus and lionheads.
Mixing other species with goldfish
Mixing other species of fish, such as tropical fish, with goldfish is also not recommended. Goldfish tend to be hardier than most tropical fish varieties. They usually do not require a heater in the tank and prefer cooler water temperatures than that of tropical fish, which require temperatures consistently in the 70s to remain healthy.
If given enough time, goldfish will grow significantly larger than most tropical fish. They can also become territorial and aggresive towards other fish species.
The only other fish species that can be safely mixed with goldfish are koi, but this is only possible in a very large pond.