Oxygen (O2)

Oxygen enters the pond by the process of diffusion at the surface and at the same time carbon dioxide is released from the pond. If plants are present, the process of photosynthesis causes oxygen to be released into the water, although this is reversed at night when oxygen is absorbed by plants and carbon dioxide is given off. Oxygen levels are dramatically affected by temperature - the higher the temperature, the lower is the dissolved oxygen content of the water. This creates a catch 22 situation as your koi most need oxygen in the hot summer months just when the water is least able  to hold high levels of oxygen.