This is just to add to what was said in a previous thread. I talked to my
wife (who is a geologist like me, but also has experience with waste water
treatement) about high pH levels some here have mentioned, and she pointed
out that high pH can also be an indication of high waste loads in the pond.
If the bottom of your pond (or rocks at the bottom of the pond) is covered
with black sludge and/or rotting detritus, you might want to try to clean
that up, and do a partial to full water change to see if that corrects the
problem. Just a suggestion.

George