Pretty much what I would expect from a pond as old as yours that has had years and years of tannic oak leaves and pine needles falling into it. Plus, all the aquatic vegetation rotting in it over the years. Throw in the soils in our area tend to be on the acidic side to begin with and there you go... Going to take a lot of lime to bring up the pH. A big question is just how much turn over does your pond have? In other words, in say a 1 week period, what percentage of the pond's water volume gets replaced by incoming water sources on average? If there is a very large turn over rate, it may be extremely difficult to bring up the pH. That low pH sure makes those BG that pretty darker coloration though.
