Here is my take on the pH. It is a value to indicate how much hydrogen ions on a solution or water in our tank. Ion can be with negative sign since we are dealing with electrons. Deducting something from a negative becomes positive. The "power" means the logarithm from the formula you showed (pH= -log[.0000001] which is the same as -log[1 X 10-7]). When the pH is 7.0 it only means that the number of ions is equal to 10 to the power of 7. It becomes acidic when it is below 7 and that is when your algae proliferates. The alkalinity (dKH
) value has an influence for the pH stability. Where do hydrogen ions come from? The simple answer is from any element containing it such as H2O. Where does oxygen comes from? The same answer as the previous one such as CO2
EDIT: This is why the pH level goes up when you have more oxygen available in your tank.