Hello Smoknjo,
the situation now is :
day 0 : i make my nutes solution with 2 liters of water and 3,2,1 ml of GH flora series. Add some acid to get a ph solution to 5.5. Put the plants with bubbler in it.
day 1 : PH is 8 or even more. I add acid to lower the PH to 5.5
day 2 : PH is 6 and stay stable to day seven

I tried the same without putting the bubbler, without air stone nore plants. PH is stable allover the week.

Maybe the air makes the ph to increase at the first moment of mixing? Chimic reaction ? I don't know. Anyway the results are not very good regarding the growth rate. At 5 weeks, the look as 3 weeks.
Oh, i use ph down for aquariums...
And you, what are your tools, and the results, set up ?
Thanks