Hello Richard
Thank you for the fast feedback.
I have to check the temperature of the water when I'm coming home from work.
So, about 22C would be good? Correct? I'll check that.
The Lights I'm using are LED Crowlight which do not produce heat. So I think this can't be the reason.
Monday, I filled one tank (10Liters) turned the bubbler on, and fixed the PH of the water at 5.5
When I checked the PH yesterday evening, it was about 7.8! I really was shocked! There was no Nutrition or something else in the Water...
When I made this first Tank (my first DWC steps) I made a big mistake. I took the wrong Nutrition and the hole water became brown. So I cleaned it all (really proper) and restarted. BUT! I don't know, if the Bubbler is completly proper. Could this maybe cause as well the extreme raise of PH? Maybe still some dirt in it?
Sadly, Hydroponics (DWC) is not that common in Switzerland (as many other things), so there's no big chance to find a Club arround me.
And yes, I got up this morning very early.
Best Regards and once again thank you for your tips.

Steven