Freshwater Fish Compatibility for Ponds

Ponds are a whole separate branch of fishkeeping, a very different animal from the aquarium hobby. Still, some fish make good roommates and some don't. When considering compatibility for outdoor pond fish, bear in mind that many of the same rules and guidelines apply as when keeping fish indoors, but you have to factor in a few new variables when planning your pond.

  1. Temperature

    • Tropical fish cannot overwinter in much of North America.

      When picking you fish, first consider temperature. The vast majority of fish in pet shops come from the tropics. Most of these fish cannot survive temperate or even subtropical winters, like those found throughout much of Europe and North America. However, goldfish and koi hail from such a climate and can live outside year-round in these regions. Additionally, some minnows -- like rosy reds and fathead minnows -- can live in outdoor ponds. Tropical fish can survive in outdoor ponds, but only if they have a powerful heater or you take them inside as the temperature drops. Never mix tropical and temperate fish, since tropical fish can carry diseases that can infect temperate fish, and vice versa.

    Size

    • Never use feeder or bait minnows as pets; they often carry disease.

      You also need to take into account the size of fish when figuring out if they can safely share an aquarium. Keep in mind that big fish -- even big peaceful fish -- will usually eat smaller fish. For example, goldfish may get along with rosy red minnows when the goldfish are juveniles. But once a goldfish reaches its full size -- potentially one foot in length -- those minnows will find that the roommate dynamic has become more of a predator-prey relationship. Also, make sure your fish will not outgrow your pond. Goldfish need about 30 gallons of pond capacity per fish to thrive. In overcrowded ponds, fish get sick and algae blooms more often.

    Demeanor

    • In North America, sunfish fill many of the same ecological niches as cichlids.

      You also need to consider the demeanor of the fish you wish to keep. Goldfish and koi will usually get along swimmingly, but some native fish will be aggressive. For example, some ponds feature native fish, like sunfish or bass. These fish can get as aggressive as cichlids in the confines of an aquarium. Even some larger catfish may get temperamental in ponds.

    Water Chemistry

    • If you keep piranha in a pond, they need acidic water.

      Most pond fish are less demanding in terms of water chemistry than aquarium fish. The larger volume of water makes the water of a pond cleaner than an aquarium, so fish adapt more easily to other parameters. Still, if you keep unusual pond fish, like tropical fish in a heated pond, you may have to consider pH or hardness when considering tankmates.