You're in coco, shit happens fast growing hydroponically. It looks like your problem is on the higher leaves only, going by the pics. The newest fan leaves basically, the bottom ones look relatively ok in comparison, yar? So you're dealing with an immobile element. Meaning unlike say nitrogen which is mobile, the plant can't pull from its resources and steal some from lower down the plant, from the 'sacrificial' leaves let's say. Now there are more than a couple of plant immobile elements but since I think you said you're using hard water, I'd start with ruling out Calcium and Iron, both of which are more likely to be in the water equation. You'll find these in
CalMag etc so that's WHY I'd start with the
CalMag. Don't need a lot, 50ppm works for me. I mentioned it in another post recently but I like to use Flower nutes when mixing
CalMag into the feed as most of these supplements so far as I'm aware contain around 2% Nitrogen. The 2% is whatever but you're in Veg and I'm assuming you're using veg nutes so they'll be getting plenty N anyway if you're feeding modestly. Flower nutes contain less N but you're topping it up with the
CalMag so it's all good.
Everything here purely drawn from my experiences, except how elements work, some other guy discovered all that :D
Keep in mind also that Calcium is such a critical element to plant life. When freely available to the plant it helps everything from water and nutrient uptake, strengthening the vascular system and cell walls of your plants and generally just makes everything happen more efficiently, almost the oil of the works if you like. It may well be a Calcium deficiency but since lack of it blocks other elements it may show up as something else.
CalMag is not necessarily something you use as standard unless you know your water is seriously lacking in Calcium or you're using RO water. But it's a must have in your armoury if you're growing in coco.
You can see in my grow log that very recently I had a deficiency and I have to confess that although I was asking the question 'what's causing the deficiency', I was getting a second opinion because I'd already made up my mind what it was. I came to a decision about what I was gonna do cos I'd spent the last 3 and a half years fucking up plants and getting a little better each one after. I'm having to take this shit mega serious because my future success and fulfilment pretty much depends on me becoming hot shit at growing. I'm completely nowhere near that yet by a long shot, and I can't imagine a time when I'll finish learning about this plant. But I know for damn sure that since I've chosen to do so I have to sit down and learn the elements and why they affect the plant the way they do. It's a complete pain in the ass my friend but especially since you've decided to grow in coco, you have a lot of homework to do. Honestly I don't mean that in a dickhead way, but in my experience, you can come on here and ask a question, people will set you in the best possible direction, but it's mere speculation because only you the grower know your plant as an individual and its full history. Plus it's a BOSS feeling when you diagnose a problem correctly and fix it yourself. :D hope I helped