Hot weather slows down these processes in a number of ways. Warm water holds less oxygen than cool water. As the water temperature rises, a fish, being a cold-blooded animal, experiences a rise in its metabolic rate, increasing its need for oxygen at the same time that less oxygen is available. Cloudy days slow down photosynthesis, in which aquatic plants are in the process of to make oxygen, making less oxygen available. If the tank is too heavily stocked, fish can run into oxygen debt even in cooler weather.
Hot weather can even bring about oxygen debt in moderately stocked tanks.
If you notice oxygen-starved fish floating near the top. A stock tank owners who have a motor-equipped boat can easily and cheaply counteract oxygen depletion. Just back the trailer into shallow water and leave the motor running in gear until the fish recover. The submerged prop will move enough water about to cause oxygen levels to increase. If you don't have a trailer, then lodge the boat against a stump or in deep water against the bank.