I have a central ac but its only on for about 4 months of the year. We live in Maine so its a cooler climate. We have the windows open. The room was built around the tank so I designed the tank out of sun and to have plenty of ventalation. This is just weird after 2 years it shifted. I just figured that maybe something was depleting like cal or mag. kh is 12. and that is what its been forever. I do test and dose and perform monthly water changes. I change 50 gallons that is about 20 percent 1 time a month if I need to i dose in the middle of it. I just figures this was strange.