PH issued in DWC system

Morning,

I am currently in week one of my flowering and am having PH drops in the evening. Quick set up info, I am running 2 20 bucket black bucket system (5 gallon) with each one having 1 5 gallon brain (res) My temps when the lights are off are between 68 to 71 and when the lights are on temps are between 76 and 82. My humidity is currently around 30%. Now my last measurements were taking at 1 am and check again at 830 am this morning.

1 Am

Sys 1 was at 6.0 PH EC @ 1.4

Sys 2 was at 5.8 PH EC @ 1.6

830 am

Sys 1 is at 5.8 PH EC still @ 1.4

Sys 2 idropped to 5.5 still @ 1.6 EC

anyone know what the issue could be? any remedies?

I am currently using H&G as feed with open sesame added, thinking about adding cal mag but haven't done so yet.

about 2 out of my 40 girls are showing small signs of curling in the top leaves.

From what I have read and witnessed from my own plants are a downward PH drift in flower is suppose to happen as Veg will be the opposite with a rise in PH.

its because of the HIGH EC. 1.4-1.6 and your 15 gal rez for 20 buckets, you need at least 40 gal for 20 buckets, with that small of a rez. you will be dosing every few hours to stay where you want it.
Daddy