It's definitely a nutrient DEFICIENCY. Either you are overwatering and this is allowing less oxygen to help the roots absorb the nutrients, or you aren't feeding enough... or as GK pointed out pH fluctuation. This can be caused by all sorts of things. Check the pH of the run off.

Knowing your ppm's or EC is essential to successful hydro growing. Plants require a minimum amount of nutrients per stage of growth, if you go in too low the plant will not be able to grow to it's full potential.

When you grow hydro you have to supply ALL the nutrients the plants need. The only way to adeqwuately measure that you are doing this is to buy an EC/PPM meter.