Sometimes referred to as an algae bloom, Another factor which can influence the pH of the water is the presence of plants, most notably if there is an algae bloom in the pond. During the hours of daylight, plants produce nutrients directly from carbon dioxide dissolved in the water and the energy obtained from sunlight, a process known as photosynthesis.
As a consequence of photosynthesis the plants utilize carbon dioxide in the day time and remove this from the water causing alkaline carbonates and bicarbonates to predominate in the water and the pH to rise. In the case of heavy algae blooms, the pH of the water can fluctuate quite dramatically through a 24 hour period.
While many large fish can survive these fluctuations, small fish can become quite stressed by these rapid pH changes.