Bottom Leaves started to turn yellow

[Mullet]

Bottom leaves turning colour

I'm a new grower for hydro, and I noticed some of the bottom leaves of each plant are turning colour.
They are about 5 weeks in the DWC
The ph is around 6.2 most times
They're light schedule is 18-6
I use General Hydroponic nutrients.

what could be the issue and and How do I correct this before it veco.es a bigger issue. Thanks

 

[Zill]

Mullet,

Plants look great. It’s not N2 deficiency. As your plants mature the canopy will densely shade the lower older original leaves. Your plant is beginning to sluff off parasitic leaves. Normal. Unless of course the yellowing goes systemic. Doubt it, the plants look great.

Zill.