With high alkalinity - total hardness there is a high buffering capacity to resist 'swings' of pH. pH will swing up and down depending on sunshine (pH up); night (pH down). At night plants produce CO2 which converts to an acid (carbonic acid) to lower pH. In sunshine carbonic acid is converted to carbonates pH goes up. The amount of buffering and plant activity determines how much the shift will be. Significant shifts of pH are not a concern in your pond chemistry. With your hardness, I doubt very much you will ever see a pH decrease to 7.0 in your pond due to even abundant submerged plant growth. In low alkalinity water expect greater shifts of pH.
Bill, I have the opposite problem: low alkalinity (37 last I checked) and low pH (around 6, even with lime on the bottom).
My pond is new with little or no plant life. Would water plants help me with the alkalinity/pH?
