Hello DonHugo, This is a magnesium deficiency. The leaves start to turn light green or on the veins and edges of lower and older leaves. This is because the plant is pulling magnesium from lower leaves to feed new growth.
Often with this deficiency, the magnesium is present, but the roots cannot absorb the magnesium properly due to an improper pH value.
Therefore it is very important to maintain the correct pH and make sure the pH does not get too low and acidic in order to avoid a magnesium deficiency.
To solve this problem, keep the pH from your nutrient solution around pH 6,5
You can also add a Cal-Mag nutrient but please keep in mind that the pH value is most likely the issue here.
Happy Growing Buddy! 💚