Tap can vary that much also, but testing the tap and the tank for KH is wise weekly.

If you use pH/KH measurements, do not assume the KH is stable.
If you run low on CO2, the plants will attack the KH as a carbon source as will algae which will compound the problem even further.

I watch the plants, which your were doing. They generally will not pearl much, growth rate slows etc. That's low CO2.

Regards,
Tom Barr