I had similar readings recently in my main tank that has been set up for about 6 months. I had a constant reading of 8.2 - 8.4, untill Sunday when i checked, and it was around 7.8. It was suggested to me that he Co2
levels in my house may be elevated. I checked my house and it turns out that we have been running the AC
with the outside vent closed for several days, and now windows open. I opend the windows in my livingroom where the tank is and viola, got an 8.2 reading last night. Hope this helps.