What could this be?

you are hydro, so it will be wise to learn what you feed in a more resolved way. It would make fixing this problem much easier and allow you to more quickly and efficiently make adjustments. manic botanix has an online nutrient calculator. use that with your guaranteed analysis labels and calculate per molecule and total ppm of your fertilizer mix. After that it'll be eaier to determin if this is a tox or a deficiency. 650-750ppm is a good ballpark for a safe mix that won't burn a plant... some may 'ask' for more and you simply react to those oddballs. vege phase - and bloom you'll want to knock down N a bit after 4-5 weeks. the plant still needs it, though. anecdote -- if i don't dial back N, it takes several weeks before a symptom becomes present, so it doesn't take much to avoid. N - 120-140 P - 50ish K - 200ish The rest varies a bit due to tap water Ca - 100-120? Mg - 80-90? S - 100-120? autos probably better 10% less and around 120-130 N to start. maybe a 650-ish total ppm of mix... plus your tap water, which will cause some variation in what is needed for your context. so many other factors are invovled, that these numbers are not meant in a precise way... but you will quickly find a safe upper and lower range of each if you observe the plant and adjust. The relative ratios are solid, but the overall concentration may change... some strains may want something different, but it won't happen often with a good balance to start.