if you are doing hydro then don't take my guess of Mg deficiency at face value. I mostly only said that because that is a very common mistake for soil growers who think they are doing a good thing by using more expensive RO water when there is no reason to.

It could still be Mg, but I don;t know hydro at all, so you should probably get the opinion of a hydro grower... i'd sugest you fill out the troubleshooting form and post in the plant problems section.