Though disease or parasites might be a problem, the most likely cause this time of year is oxygen depletion. Furthermore catfish ponds are those most likely to suffer fish die-offs from this problem. This is because catfish ponds are more likely to be intensively managed and stocked with high numbers of fish. Hot, still cloudy days cause oxygen levels in tanks to decline. Ordinarily, aquatic plants, mostly single-celled algae, produce enough to maintain oxygen levels.
Wind also helps aerate the water.
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.