pH drifting downward in a fertilizer reservoir.

I started running into CalMg problems on this last run Which I attribute to low pH. This is in soil. I started mixing up 10 gallons of fertilizer solution in a reservoir with an air stone in it to keep it from getting stagnant. After mixing the fertilizers I adjust the pH to 6.7. What I’m finding is that the next day it’s at 6.0. So I adjust it again and the next day it’s at 6.3. So now I’m adjusting before each watering. I’m wondering if the air stone is causing the CO2 in the air to form carbonic acid. Any thoughts? I’ll switch to a water pump to keep things mixing and see if there is a difference. I’m using GH fertilizers with some B52 from AN and some cal mg.

Does the EC climb also? Evaporating perhaps?

No. EC stable. it’s covered.