Tropical fish owners must keep the water in the tank from getting too warm or the fish will die. Why is this true?

Tropical fish owners must keep the water in the tank form getting too warm or the fish will die. This is true because the fish that as grown in an environment at mild temperature got used to that environment. So if there is a constant increase or decrease of temperature, even if at one degrees Celsius, overtime the fish will die. For them, their environment has become polluted because of the change in temperature. It unbalances the system of living they used to have.

Tropical fish usually live in warm water so the water temperature has to be the same in the tank.