About 10% of homes in the United States have aquaria, and purchases of live "ornamental fish" for aquaria and ponds amount to more than $600 million a year in the US.⁽²⁴⁵⁾ A single large aquarium supply house may market 3,000 distinct breed lines. Tampa, Florida, is the major port of import for the United States, doing about $7 million per year.⁽²⁴⁶⁾ The greatest proportion of these is "tropical fish," both freshwater and saltwater. The majority of these are imported from countries such as Thailand, Indonesia, Singapore, Hong Kong, and the Philippines.⁽²⁴⁷⁾ Many other tropical fish are cultivated in Florida and other southern states for both domestic and foreign sales.

The term "tropical fish" is sometimes used as a synonym for aquarium fish. Although it is much smaller, there is also a significant trade in "coldwater" fish (all freshwater), some of which are deliberately used for stocking ornamental ponds in temperate climates. For example, aquarium supply houses advertise the Japanese Colored Koi (Cyprinus carpio), a coldwater freshwater fish recommended for use in either large aquaria or ponds, which is most active in temperatures of 10°-23° C (50°-74° F), but will also survive in ponds frozen over on the surface.⁽²⁴⁸⁾ More generally, fish are highly varied creatures. Some species double as both "tropical" and "coldwater," although it might be few if any of these which would survive in a frozen-over pond. The goldfish, now common throughout North America, was an ornamental fish release from China. Other aquarium releases into the Great Lakes include the bluespotted sunfish, snails, crustacea, and a number of particularly noxious aquatic weeds.⁽²⁴⁹⁾ With the continuing increase in the popularity of exotic fish for aquaria and ponds, there is no reason not to expect such invasions to continue.