Nitrogen if anything for adding nutes, but you really should test the pH of your runoff water before you do that. If it's outside the 6-6.5 range, you should alter it to match that, and it'll fee up some of the nutes that you've already got in there. The other reason for yellowing is that it's not getting light to them, and if that's the case you might want to try tying it down to expose those ones to more light. Good luck! Oh, and you can defintely switch to 12/12 now.