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: Fish that prefer water temperatures 21ºC and above usually grow faster than those in water 19ºC and below.
reason: The context explains coldwater fish as living at 20ºC and below grow slower than warmwater fish.