The thought of goldfish often brings to mind the image of a solitary fish in a tabletop fishbowl. There's a reason for this stereotype: goldfish don't play well with others. Pick your goldfish's tank mates with care to keep them from entering attack mode and ruining the tank's tranquil atmosphere.
Goldfish prefer to swim in still water, which is why an unfiltered fishbowl works well. They usually avoid the current surrounding a filter in a larger tank, and they don't like to swim around fish who make large waves. Bottom feeders such as a cory are a good option, as they don't like to make waves.
Goldfish aren't known for their fast swimming skills, although they can be aggressive when you pour fish food in the tank. Choose fast, but small, swimmers that can compete with the goldfish for the food without disrupting the water excessively. White cloud minnows are a good choice, although their small, 1-inch size can look tasty to larger goldfish. Use the minnows in tanks with smaller goldfish varieties to keep them from becoming the goldfish's next snack.
Aquatic creatures who eat algae, such as apple snails, plecos and loaches, can live with goldfish successfully and help keep the water clean. The trick to tanking algae eaters with goldfish is to feed goldfish floating food on side of the tank and feeding the algae eaters with sinking food on the other end of the tank. This eliminates competition and provides enough food for the goldfish companions to keep them from attaching to the goldfish; they can be attracted to the protective slime coating on a goldfish's scales. If they remove some of the coating, the goldfish becomes vulnerable to disease. Keeping the algae eaters well fed can keep them off the goldfish.
Buy fish smaller than the goldfish and they're likely to be missing in the morning. Goldfish like to eat other fish that they can swallow whole, so a fish that's the same size as the goldfish is a safe bet. Also, aggressive fish can be attracted to a goldfish's shiny fins, leading them to nip at the fins or eat the goldfish, so avoid those in your goldfish tank.