Premise: Coldwater fish, in the context of aquariums, refers to fish species that prefer cooler water temperatures than tropical fish, typically below 20 °C . Some examples are koi and goldfish. These species tend to grow more slowly and live longer than fish that live in warmer waters, and are generally felt to be easier to keep.
Hypothesis: Although coldwater fish are generally felt to be easier to keep, they also prefer to live in waters that are cooler than 20*C.