Has the water company been changing the water?
Have you been topping off evaporation loss with tap water?
Please test and post:
GH, KH, pH of tap and tank.
Repeat the pH test on tap water that has sat out 24-48 hours.
I don't think the water company has changed the water.
Yes, I top off tap water. The GH of tap water is so low I decided to use it. Tank GH has been historically 5 dGH.
I use Equilibrium with water changes, and I use Seachem Flourish Comprehensive.