"Cold-water" is a term used in the aquarium community to refer to fish that live and thrive at low temperatures, usually below 20 degrees Celsius (20ºC). This differentiates them from warm-water fish, which require a heated tank. Cold-water fish grow more slowly than warm-water fish, and also tend to live longer. For these reasons, cold-water fish are considered easier to keep. A vast number of cold-water species is available, so diversity of size, shape and colour --- even within the same aquarium --- is entirely possible.
This fish is the only cichlid native to the United States, originating in the Rio Grande region of Texas. Its natural habitat includes freshwater rivers and other waterways, particularly those with deep, sandy bottoms. It is commonly found in the aquarium trade, and can be successfully kept with other large aquarium fish. However, the bonded adult pairs will become territorial and aggressive after mating and producing eggs. This behaviour makes Texas cichlids a challenge if they are allowed to breed. They are not considered a beginner's fish.
The rosy-red minnow emerged from breeding stock of fathead minnow baitfish and is used most commonly as a feeder fish, though it is also popular when kept in large, social schools. It grows to a maximum of 2 inches, and can be kept in either an aquarium or a pond. Much of its success can be attributed to its hardiness; rosy-reds can tolerate high temperatures and low oxygen levels, as well as slow-moving or stagnant water. In addition, their primary use as bait and feeder fish makes them very inexpensive for the beginner.
Originating in China, this pet cold-water fish is distinctive for having the fewest modifications in relation to its wild ancestor; only its colour has changed in the thousand years or more that it has been bred in captivity. Common goldfish can be almost any shade from red to blue or brown, and can also be found with very beautiful metallic patterns on their scales. They are hardy, social fish which are best kept with other fish of the same species. Common goldfish can also become tame over time, and are one of the few fish species that can be taught to eat directly from the feeder's hand.
Pygmy sunfish are native to the southeastern United States. They inhabit weedy, slow-flowing water, such as drainage ditches, swamps and small streams. Males are more brightly coloured but slightly smaller than females. These fish grow to a maximum of an inch in size and are quite at home in aquariums containing real plants. However, they tend to be picky eaters who demand live food such as blackworms. They are also considered to be good tankmates for other fish, provided those fish are not large enough to devour the tiny sunfish.