Yellow leaves are a symptom, not a diagnosis. In hydroponics the most common causes are a nutrient imbalance or a pH that is outside the range plants can use. Check the pH first, then look at where the yellowing appears.
Older leaves turning yellow
Yellowing that starts on older, lower leaves often points to a shortage of nitrogen. Check the nutrient strength and whether the solution has been topped up with plain water for a long time.
New leaves turning yellow between the veins
When new leaves are yellow but the veins stay green, iron is a common suspect. A pH that is too high can stop plants taking up iron, even when it is in the solution.
What to do
- Test and adjust the pH.
- Check that the nutrient is mixed to the label.
- Replace the solution if it is old or heavily topped up.