old dan leaves dying where new branches growing.

My plants are 6 weeks old. Using foxfarm ocean Forrest soil and nutes to their schedule. They are under 1000 Watt mh lights. Co2 in room 1500 ppm Ave daytime. Nutes every other watering at 1200 ppm and 6.2 Ph. Temp is about 80 day and 72 night Ave.

I am growing 9 strains and on almost all of my plants the big older fan leaves are dying. It doesn't start until a new branches start to grow at the site where that leaf meets the branch and progressivley get worse as the branch gets bigger. I am assuming the new branch is using nutes from the leaf. I cut the leaves when they are more than 50% dead.

Is this normal? Sorry no pics. First grow.

Thanks all

my plants did't turn yellow til the end of harvest got anay pics of your plants ?