Now that you mention caliber the PH probe, I may need to replace it. I'll try that.The first step is properly calibrating the PH probe.
I would increase tank aeration (get heavy ripples on the surface of the water, use your skimmer.
If that doesn’t work, open some windows and see.
Use kalkwasser or 2 part with sodium hydroxide for the alkalinity part.
CO2 scrubbers should work. A combination of the above should certainly raise your PH.