Dave,
During the night plants undergo a process called cellular respiration. Because sunlight is unavailable for energy, plants will respire in order to obtain their energy from carbohydrates stored from previous photosythetic activity. During the process of cellular respiration carbon dioxide is given off. This carbon dioxide depresses pH, sometimes quite a bit. pH levels drop and drop throughout the night until sunlight is available to start photosynthesis. During the day pH levels will increase. This pH cycle can change pH values by as much as two full points during the day. This is important because ponds that are artificially fed can have higher levels of ammonia. Ammonia comes in two primary forms-- ionized and unionized. When water becomes more acidic (lower pH) there are more hydrogen ions available to bind with NH3 forming NH4+ which is the ionized form. This is less dangerous to fish. When water becomes more basic (higher pH), fewer hydrogen ions are available and NH4+ loses hydrogen ions to the water and reverts back to NH3 which is the unionized for of ammonia. This is extremely toxic to fish in high enough concentrations. Bottom line is that high pH creates an environment with more dangerous unionized ammonia and fish can stress or die.
An accurate assessment of your pond's pH would involve testing at various times of the day, in particular at 1 hr. before sunrise and late afternoon to establish how wide your pH range is.
