Is a goldfish warm-blooded

it is best not to mess with the temperature of the water... Goldfish like solid temperatures, it is best to keep the temperature around 75-78 degrees.

They are exothermic animals relying on the temperature (thermal energy) of the water to stay alive. Too hot and they run out of Oxygen and suffer from exhaustion and eventually die. This is because gases are more soluble in colder water. Too cold and their vital organs slow down and stop working and the fish dies. For Goldfish the best temperature is 68-72 degrees Fahrenheit.

Goldfish, are, however more active in warm water, because the heat from the water increases their metabolisms. This actually decreases their lifespan and is dangerous.
Goldfish are more active in warm water than cold.