For buffering have you tried something more natural like oystershell chicken
grit or small limestone rocks? I found neither one raises the PH much but
does buffer the water and keeps the PH from dropping. Both keep the water
above 7.

What is oystershell chicken grit?

Limestone rocks is a good idea. I imagine it will take some time to
equilibrate, but once it does, it should stay pretty solid. I'd be
really happy if I could manage to keep the carbonate hardness up
without the pH going so high.

Thanks!

Joan