Photosynthesis is the process by which plants convert the sun's light into chemical energy. Photosynthesis increases in periods of heavy sunlight. Plants absorb carbon dioxide from the pond to help produce sugar to facility the photosynthesis process. As the carbon dioxide and carbonic acid levels decrease in the pond, the pH levels rise. Pond pH is highest a few hours before sunset. In some instances of photosynthesis, all the carbonic acid can be removed from the pond, causing a pH level above 9.
Photosynthesis stops after sunset, but respiration continues. In respiration, a plant takes in oxygen and releases carbon dioxide elevating the carbon dioxide levels in the pond. Carbon dioxide blends with the water, creating carbonic acid. As the carbon dioxide and carbonic acid levels rise, the pH levels decrease in a pond at night. Levels of pH in a pond can be lowest two hours before sunrise. Natural occurrences that affect the carbon dioxide levels also affect the pH levels in a pond.
A fish excretes ammonia through its gills into the pond water as proteins metabolize in its body. Ammonia is affected by pond pH levels and is more toxic at high pH levels during the day and less toxic with lower pH levels at night. Ammonia toxicity increases with high levels of sunlight and high temperatures. The continuing cycle of photosynthesis and respiration will cause the toxicity of the ammonia in a pond to rise and fall.
The continuing change in pH levels in a pond is stressful for fish. Dissolved salts like magnesium and calcium can help to balance the pH and buffer the pond against the changes cause by a variety of environmental factors. The salts act as a sponge and can lessen the effects of carbonic acid on the pond pH levels. The salts act as a buffer, stabilizing the changing pH levels.