Magnesium deficiency causes the plant leaves to yellow and curl at the edges. It can be prevented by keeping the temperature between 21 and 23 degrees, the pH between 6 and 7 and feeding the right amount of magnesium
If your grow system is short of iron then problems will first appear at the top of the plant. The younger leaves will appear yellow and bleached with the veins remaining their usual green colour. If this becomes severe then the leaves can die off.
Any plant growth system needs a good balance of nutrients for a strong foundation and a prosperous growth. It is essential to know from the outset what is required and what can go wrong if the ratios are not ideal.