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.
Massachusetts Water Resources Authority data:
GH: 13 mg/l /17.86 = 0.73 dGH
KH: 41.7 mg/l /17.86 = 2.33 dKH
GH: 5 dGH
KH: 3 dKH
pH of top-up water to follow