Coldwater fish

Coldwater fish, in the context of aquariums, refers to fish species that do not require a heater to remain within tolerable temperatures in a typical indoor aquarium. Some examples are koi, goldfish, weather loaches, hillstream loaches, many species of barbs, sunfish of the family Centrarchidae, many Central American cichlids, and a variety of other species. Some, but not all, of these species are capable of surviving in very low temperatures, including ponds, lakes, or rivers that freeze over winter.

Japanese koi carp are coldwater fish.

It is important to note that just because these fish are capable of surviving in unheated aquaria, their temperature preferences may vary. For example, koi and goldfish are commonly considered to be cold-water fish because of their ability to survive at very low temperatures, but their temperature preferences are 32°C (89.6°F)[1] and 28°C (82.4°F),[2] respectively. Because many of the ornamental fish considered to be “coldwater fish” are more accurately eurythermal fish and many prefer temperatures similar to, or even warmer than those preferred by certain tropical fish, the term “coldwater fish” in the aquarium context often misleads pet owners into keeping fish below their preferred temperature.