Rusty color on viens week 5 flower

I have topped my resivor off with plain tap water that didn't set 24 to 48 hours and Temps ate 73 to 84. Just hooked smooth exhaust fan up to do temp a lil. Good fan hydroponics feeding schedule. Mammoth p, kool bloom, calmag.. It starts on the middle vien toward the stem and it's reality the only place I see it's on anti leaves so far anti help would be greatly appreciated. Thanks guys

Could be something as minor as sun/light burn or tanning. Don’t change or adjust too much, it is a plant, keep things steady and mild and monitor.