I would love to keep it around 8.5 but I don't seem to be able to do
so. I can't figure out where the excess base is coming from. I
haven't added any cement blocks or anything like that. A long time
ago I added some CaCO3 and some dolomite lime, but the pH never seemed
to increase. Suddently this spring it started going through the roof.
This was before I added the CaCl2 or the BS. My GH was very low, and
my kH around 100. I did a major water change, which dropped the kH
down to about 50. At that time, I was still having high pH values in
the late afternoon--around 9.2, But the pH values in the morning was
down around 8.1, so I was getting huge diurnal pH swings. I was
worried about the effect of the wide swings on the fish. I know that
calcium is supposed to precipitate the excess carbonate out and was
hoping that would dampen the afternoon pH rises.. I also know that
excess photosynthesis will raise the PM pH. So that is why I was
doing what I was doing and trying to keep the algae under control. I
can do a water change, but I'm afraid I'll be right back there where I
started unless I let the KH get dangerously low. I am worried about
alkalosis, but I am also afraid of setting them up for a pH crash
overnight or dangerously high pH swings.

Joan

9.3 is quite high. You should try to keep it around 8.5, highest.
Baking soda stops dissolving in that range, so that shouldn't be
kicking it up over 9. Have you put any objects in the pond recently?
Any cement blocks or something like that? That will raise your PH
through the roof.

If your PH stays up in the 9s or higher, you'll run into Alkalosis,
which can cause big losses. You'll notice that the fish get
excessively slimy and are gasping at the surface if that happens, and
you won't have much time to get things right, starting with a major
water change.

If it were me, I'd avoid the CACL2 and baking soda at least until you
get this worked out. It will only make the problem harder to diagnose.
The PH is likely to be a much worse problem for your fish than GH/KH
levels.

Dave