I use koi clay but usually for algae and I don't think it affects the PH but it is supposed to remove harmful minerals. I'm wondering if Microbelift Autumn Prep might be better?

It depends on how much calcium bentonite clay you add and which brand you purchase or have. If you go by the recommended dose for the clay it will have no effect at all on the pH. Some of the calcium bentonite products have a high pH and some have a lower pH. I think it is best to have one with a pH between 7-8, but if I had water source that had a low pH I'd use one with a high pH.

Calcium bentonite doesn't really collect suspended particles it actually magnetically attracts and absorbs toxins in the water. It also replaces minerals and trace minerals that have been leached out of your pond water and adds some that may have been lacking in your water source.

Thanks jalal and gene, we had about 3 inches of rain so maybe things are looking up for us down here. I think I will try some of the low ph clay. Sounds like it will help the water quality if nothing else.