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The term coldwater fish can have different meanings in different contexts. In the context of fishkeeping, it refers to ornamental fish species that tolerate the temperatures of a typical indoor aquarium well and do not require a heater to remain active, as opposed to tropical fish whom need a heater to survive in the room temperatures of temperate climates;
In the context of ecology and fishing, it refers to fish species that prefer to inhabit waterbodies or depth zones with much lower temperatures than the average temperate water. Salmonids (e. g. salmon, trout, char and graylings) are a classic example of such types of fish.

Most or all ornamental fish species are able to tolerate temperatures as low as or lower than room temperature, with most stenothermic tropical species having critical thermal minimums of around 10 12 °C. Although these fish are capable of surviving in unheated aquaria, their temperature preferences may vary. For example, koi, goldfish, and pond loaches are commonly considered to be cold water fish because of their ability to survive at very low temperatures, but their temperature preferences and/or physiological optimal temperatures are 32 °C (90 °F), 24 31 °C (75 88 °F), and 26 28 °C (79 82 °F), 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.

The term "coldwater" is also used to refer to wild fish species that prefer bodies of water that are colder than most temperate waters. In recreational fishing, anglers may loosely break down fish into categories of warm water fish, cool water fish, and cold water fish. Warm water fish, such as largemouth bass, sunfish and bullhead catfish, are species that tend to dwell in relatively warm tropical and temperate waters similar to the room temperatures that humans easily find comfortable. Cool water species, such as smallmouth bass and walleye, can tolerate a wide range of temperatures, but tend to be most abundant in cooler rivers or deeper parts of ponds and lakes, where the temperature is slightly lower than room temperatures. Cold water species, such as salmonids (e. g. salmon, trout, char, graylings, freshwater whitefishes, etc.) and gadiforms (cods, hakes, pollock, haddock, burbot and rocklings, etc. ), however become stressed at warm temperatures and are most active in colder temperatures around which resemble a more subarctic or alpine condition. Because these designations are informal, different fisheries management authorities may recognize different boundaries in temperature preference between the categories.