DWC: Brown spots and pale leaves on the 2nd week

I'll give her a res change soon if it gets worse, let's wait and see. Again thanks for your input!

I get a lot of bubbles in the res from 2 pumps and PH is quite stable between 5.8-5.9 with this last res change for 7 days now, actually the most stable I could get than my entire first grow. Perhaps this also could be the reason, the PH is not swinging to above 6 and not all minerals are being absorbed - although I heard stable PH + EC means you are spot on, don't know if this is true. Or a lockout due to the heavy feed (I'm actually doing 1/3 of GH recommendations). Thanks a lot for the tips. I'm using Armor Si + Diamond Nectar (did not write it the question due to text size limits), but I cannot find Hydroguard here.