Under sunny conditions and moderate temperatures, aquatic plants, mostly single-celled algae, produce enough oxygen as a byproduct of photosynthesis to maintain oxygen levels in ponds. Wind, or the lack of it, is also a factor as it helps to aerate the water. Summer conditions slow 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 less oxygen is available.
Cloudy days slow down photosynthesis, making even less oxygen available to the fish.
If the pond 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 ponds. Often the pond owner won't need to determine pounds of fish per acre to diagnose oxygen depletion. Oxygen-starved fish can be seen gasping at the surface or swimming weakly to the edge of the pond. Oxygen depletion will affect all sizes and species of fish to various degrees.