Ah I have seen this too, unfortunately I don't have an answer (yet? haha). From what is visible on the pix, otherwise your plants are doing great, is that correct?! Then I'd just let them go. You can get rid of your nitrogen by adding carbon-heavy material btw. (as Magdup kindly shared a link to ). But I agree with those who have said it's probably not really an N issue. One thing I learned obsessing about those curly tips is, that if the cause is overwatering, they won't be uncurling anymore. That is, your conditions may have returned to ideal, but you will continue seeing those signs that appeared when they dipped under, they won't reverse. In my case, I saw how the affected leaves were about the same age - older and younger leaves not having those symptoms, and finally concluded I must've messed up sometime and not noticed. lol maybe I was just fed up with thinking about it inconclusively Cheers!