All animals must maintain homeostasis, which means a normal body temperature, normal heart rate, pulse, blood oxygenation, and normal functioning of organs.
This means in hot environments they must keep cool; in cold they must keep warm.
Most animals have adapted over thousands of years to adapt to temperature changes.
Fur, foot pads, and even how animals find food have each undergone adaptations over the years.
God protects those fish, kid. Don't you worry about those fish.