Broken pH meter or unstable solution

Hello commrades! I am new here so i will say few words for introduction. I am now growing for 5 years, mostly outdoors and several times indoors in soil and i have never had any problems untill now. I felt confident to move on hydroponics or aeroponics so i have chosen AeroFarm. Maybe it was a mystake but this is another thread. I am using 400W MH/HPS Lamp with cooltube, several fans to cool the air, GHE Tripack(Flora Micro for hardwater) Ferro PH Down and Milwaukee PH and EC meters. So my problem is: I fill tap water in separate container (20L) add the nutes let it sit for a while then adjust the ph to 5.6, then i let the solution sit for 48 hours. For that 48 hours ph rises to 5.8-5.9. That seems normal but when i add the solution to my AeroFarm for just 8 hours of circulatin ph rises from 5.7 to 7.0. That happens with or without any nutes in the water. If i add just a drop of Ph Down the ph in the solution gets down to 5.9 or even lower. I was thinking that my ph meter is broken so i let the AeroFarm run but my plants started yellowing. What do you guys think? Where is the problem?

Do you have calibration standard fluids? Pick some up and adjust your PH meter using the little screw on the back.

You joined 4 years ago ....and just made your first post ....wunderbar!

always keep a backup you do have a backup...? until then dwc weed will survive in a ph range of 5-8 it won't like it but will survive

I have calibrated the ph meter 10 days ago, also it has an indicator that shows when to calibrate. Excuse my ignorance but is it possible for the plants in just 1 hour with adjusted ph to change their color to a little greener?