Think of pH as the way the water feels against the fish's skin. The ideal pH reading for koi and goldfish is around 7.5 but these fish can tolerate a range between 6.8 - 8.2. We prefer to use pH strips to test pond water because it not only does it give you the pH level it also indicates whether the water is buffered properly or not.
Buffered water means the pH is stable.
It tends to run higher at night then lower by morning, taking it's cue from the natural CO2 and Oxygen exchange of the plants in the water. The difference in the pH at different times of the day is called pH swing. When it is more than just a couple of points off it can be dangerous and unpredictable. The object is to stabilize the pH at an acceptable level. Now think of the pH suddenly falling.