How to Put Goldfish and Tropical Fish Together in an Aquarium

Home aquarists have traditionally kept goldfish and tropical fish in separate tanks due to their different oxygen, temperature and space requirements. Experienced aquarium enthusiasts with limited space for multiple home aquariums, though, can combine the goldfish and tropical fish in the same tank by striking a careful balance between their optimum habitats and monitoring the individual fish for signs of stress.

Install the external canister filter. It allows for a high level of dissolved oxygen in the aquarium water. As a coldwater species, goldfish require more intense filtration and accompanying aeration than tropical species, which tolerate lower oxygen levels.

Set the aquarium heater to about 70 degrees Fahrenheit and submerge the thermometer. Goldfish tolerate room-temperature water, while tropical fish do best in aquariums at about 75 degrees; find the median temperature that your species will tolerate by monitoring the thermometer and any signs of stress in the fish.

Choose your species. Start with common goldfish (they're hardier and less susceptible to stress than fancy ornamental varieties), then choose easy-to-keep tropical fish that are about the same size as the goldfish to avoid competition. Do not pick species that are too aggressive (like tiger barbs) or too shy and slow-moving (like angelfish); try to maintain a group of active, hardy species that can coexist without problems.

Select the proper number of fish by dividing the surface area of the tank in inches by 12 for tropical fish and 14.5 for goldfish. The resulting number is the total length of each type of fish that the tank can accommodate. For example, a 24-by-12-inch tank has 288 inches of surface, which can accommodate 24 inches (288 divided by 12) of tropical fish or about 20 inches (288 divided by 14.5) of goldfish. Keep in mind that you will be combining tropical fish and goldfish, so the total inches of all fish in the 24-by-12 tank averages 22 inches. Don't exceed this number or your fish will be overcrowded and oxygen-deprived.

Add the fish to the aquarium: float the plastic bags containing the fish in the 70 degree water so that the species can acclimate to the new temperature. After 15 minutes, gently release the fish into the tank.

Monitor the fish for any territorial behavior, aggression or stress for the first few days after introducing them to the tank.