it's paling from the top, so not nitrogen, that's mobile, it would turn yellow from the bottom. looks more like iron is either lacking, or more likely getting locked out. I'd check your pH with runoff, otherwise try a general liquid fertilizer. If it's a deficiency, it should clear up in a couple days, but I'd look to the pH first. just had this issue in DWC, but my issue was the pH hit 8.0, not any shortage of nutes. there's also the possibility of using a foliar to try getting the missing nutrients in there, but you'd need to ask a more experienced grower than I for advice on that one, I've never really done foliar feeding