Please help...First issue during flower...

Hey guys.

First timer here. Me and my mate got this setup going and about a month ago we ran into some heat problems. After the help from this lovely community we overcame it and were happily cruising in our 4th week(22days) of flower until........

We have yellowing on 1 out of our 4 ladies. It seems to be cropping up on a 2nd but not bad at all.

Now I know some yellowing is normal during flower but this dont seem right.

4x 600w. 1 for each plant. Drain to waste. Straight coco. Feeding GH 3 part, 4x a day for 20 seconds. PH always checked as well as ppm. Have not had an issue with nutes until now.

Check the photos. I took the worst leaf and took a photo as it has a wierd shade of brown in it.

Also please note that it does not look as bad in the 3rd photo. Those light green leaves are actually yellow. Not sure why it shows them darker...

Excellent 1st grow!
I'm not a hydro grower, but I'll get things started for the others.
They are going to want to know what amounts in ml of each of the 3 GH nutes are being added and total ppms each feeding?
Anything else added?
What type of water you are using?
What is the PH of the water when applied to the coco.
Hope we get to see harvest pics.