Needing some advice DWC

Hey folks, I’m new grower needing some advice with my plants.

I’m running 2 autoflowering northern lights from Seedsman both plants 7 weeks in from seed now under a hipargero 800w lights 20/4 lights cycle with fox farm trio nutes.

As you all will see in the photos my plant in the left is bigger then the other one don’t know why cause there are from same seed bank but I did some mistakes with the one in the right so maybe is cuz of that it is smaller anyways is not the case.

My question is, is it normal that my bottom leafs (the very bottom leafs) turn yellow and consequently died? I’ll add some photos for you all can see what I mean. Is that normal? Or should I add some more nutes? I’m running with 750ppm my tap water start with 170ppm. Fox farm trio plus cal-mag nothing more. I’m following their schedule for hydroponics half of the dose per gallon.

And I’d like to know as well if is there any problem if the top of my biggest plant touch the lights? Cause I have no more space I cannot rise up the lights anymore it’s on the limit!! I really do not expect of autoflowering to get this big I’m very impressed actually especially cuz is my first grow ever! Should I expect the plants will get even bigger? And what can I do if I can’t rinse up the lights? Don’t want my plants to burn you know.

Thanks in advance.

 

Thanks bro