Why do fish die easily when the weather is hot?

The limit culture temperature of goldfish is between 0 and 39 degrees. The higher the temperature, the lower the dissolved oxygen content of water, and the easier it is for goldfish to lack oxygen. Therefore, when the temperature is high and the fish lacks oxygen, oxygen should be replenished in time to prevent the fish from dying due to lack of oxygen. When changing water, the temperature difference should also be controlled well, preferably within 3 degrees, so as to avoid catching a cold due to excessive temperature difference.