Goldfish are active and curious fish, but they don’t need to have companions to be happy. Adding tank mates to a goldfish habitat is really more of a personal decision than a necessary one. Your goldfish will be content as long as they have a clean tank to explore, plenty of room to swim, and an abundant source of food.
Since goldfish are not aggressive, they can be kept with most community fish provided the other fish are larger than the size of the goldfish's mouth. Generally, it is best to keep goldfish with tank mates that have similar swimming abilities to ensure all fish can obtain enough food and swimming space.
Suitable goldfish tank mates for smaller aquariums may include comet goldfish, white cloud mountain minnows and danios (Zebrafish).
White cloud mountain minnows
Minnows may be a good option. They’re very inexpensive, tend to school together, and only grow up to 2 inches long. Yes, these fish can fit in goldfish mouths, but they’re very fast and nimble compared to the slower fancy goldfish and are difficult to catch. In the event that one does accidentally get eaten, it’s not harmful to the goldfish. Being cousins of the zebrafish, they have similar taste in food and care.
Danios or Zebrafish
Danios come in a variety of colors and fin lengths. These types of zebrafish are peaceful, tiny fish that like to swim in groups. They are large enough to avoid being eaten by goldfish. They are fast and active, which means they will stay out of trouble and add plenty of interesting activity to your tank.
Another goldfish such as standard comet and shubinkin
Goldfish like swimming with other goldfish. Standard comet goldfish should be kept with other comets with fancy goldfish kept with other fancies. Fancy goldfish lack the mobility and maneuvering that can be a problem during feeding time. Comet and shubunkin varieties have similar body types, but different coloration.
The Comet Goldfish is a bit shorter and wider than fancy goldfish and has a wide-spread tail with sharp points on them. These fast swimmers usually end up between 10 and 12-inches long at maturity. Trying to limit your fish's size by keeping them in a tiny tank or restricting their diet will only stunt them both structurally and immunologically. Healthy goldfish thrive by having lots of room to swim. So, no matter the type of goldfish, you will need at minimum 20 gallons per fish.
Comets are easy to feed and will eat flake, floating, and sinking foods. All goldfish should be on a goldfish-specific pelleted diet. If you have overly-competitive fish in the same tank, try to spread out the diet over the entire tank surface or mix floating and sinking pellets.