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More improtant than the pH is the KH (carbonate hardness). The KH will
prevent pH swings, or at least severely restrict them. The carbonate
hardness is the ability of the water to absorb acids without changing pH.
For the pH Down to work, it first must consume all the KH before the pH
starts to change. Once the KH is gone, a pH crash is imminent. For a good
stable pond the pH should be above 80, with values of 100 being better and
200 or 300 not being unreasonable. Filters generate acids that consume KH,
and require resonable values of pH to function. So KH must be replaced
periodically. If your tap water has good KH values, this replacement of KH
can be accomplished by water changes which at 10% per week is needed anyway.
If the KH of the tap water is not high, then the addition of baking soda is
advisable.

I haven't checked my pond water for KH. I guess I better buy a test
for that. I did forget to mention that my pond was filled with water
from my well.

Thanks for the information.

Lois