Browning between veins!!

Watered phd 7 last water was Sunday night. It was dehydrated after a few days of high heat. so just added plain water. I usually add 1/2 strength cal-mag but didn’t as I just wanted to get the water into her. 2 of the plants were treated exactly the same for Dehydration but only one plant is showing problems.

Just started week 5 flowering. Soil.

I’m thinking magnesium deficiency but I’m not 100%

Sorry about pic but the lights were on. I’ll get a better picture when it’s off

Definitely from PH fluctuations. Also add in a little more CALMAG to be sure.