OK, I'm pretty fried atm, but I'm going to try to help... take it with salt.
First I recommend getting the book
Marijuana Garden Saver: Handbook for Healthy Plants. It's by J.C. Stitch, edited by Ed Rosenthal and only costs $17.95 at whatever big book store you have nearby. I got mine at Barnes and Noble I think. This book lays out the nutrients and basic conditions needed and more importantly, tells you how to diagnose all the issues you are likely to run across WITH PICTURES. It's great.
Now don't get me wrong - there are online resources that might do you just as well for free, and if you can, use them. However, most people either can't or prefer not to take a computer into the grow room. I take the book in there with me and compare it directly to what I see. This book is a great help. Eventually, you will find you rarely use it. And that's great, but until then it is worth every cent.
OK. next, please be careful of your reactions to problems with the plants. It is very easy to over-react, over-water, and just plain over-love these plants. In hydro you have to be johnny on the spot with your maintenance, but you can change everything at a whim. In a soil or soil-less like we use, everything is less immediate. They will do OK for longer in poor conditions, but it takes longer for the effects of a correction to really show. The medium slows everything you do. Expect a week to pass before you see a change in a lot of cases. My point is that if you try to correct again before that previous correction has really had a chance you can really throw the whole system out of whack. OK that sounds a little preachy there - sorry - but I think it is important, and you seem pretty worried.
So looking through the aforesaid book, I find four candidates for deficiency:
The first is N (nitrogen). N is the usual culprit when you see yellowing because it's a huge component of the chlorophyll that makes the plant green. If you don't have enough Nitrogen, leaves will yellow. We can rule that one out though because N is very mobile. By mobile I mean that the plant can deconstruct something at point A, freeing the components to be transported and then used to build something at point B. Because N is very mobile, the plant will steal what it needs from lower, older, more shaded leaves to continue construction of the green in new growth. It makes a smart move and eliminates less useful growth in favor of the new. So when a plant has an N deficiency, the yellowing starts down low. Yours is at top, wrong deficiency.
Second is Fe (iron). With an iron deficiency the new growth goes yellow and the yellow will be mostly in the space between the veins. The veins will stay green for a long time still. Fe is needed to make chlorophyll - it is a catalyst. That means that it is needed to make a reaction happen, but it isn't incorporated into the solution. To put it another way, it's like tools when you are building a machine. You need the tools to do the job, but they aren't a part of the machine and they can be used again on another. Fe is the same with building chlorophyll. But Fe is not really mobile. Once supplied to new growth, it pretty much stays there. The expansion in a leaf involves a lot of expansion of the webbing between veins, so the Fe there can be spread out and at some point becomes too thin. Then there isn't enough to keep up with the demand for chlorophyll in the webbing and it yellows. I can't see close enough to tell if this is what you see.
Fe shortages are usually a pH problem. I think most nutrient shortages are, honestly. Most modern feeding systems provide everything you would need. I use this chart which gives a pH range of absorption for each nutrient component. In this chart the plant needs low 6s on the pH just to get a decent absorption of Fe going:
Rusty water would be an OK way to get some Fe in there, but I think a good micro-nutrient supplement would be better. I use TM-7 from BioAg, sold at my local shop. It proves the Fe along with a lot of other micronutes: B (boron), S (sulfur), Co (cobalt), Cu (copper), Mn (manganese), Mo (molybdenum), and Zn (zinc). Good stuff. Myself, I would give a feed at a pH of 6.0-6.1 and add some TM-7 to the feed.
The next possibility is Mg (magnesium), but like N magnesium is mobile, so it starts low, not on the newest growth.
The last is S (sulfur). Young leaves first. Leaves become brittle and narrow. Overall stunting of growth. Thinner stems that may become woody. This is less likely a culprit than the Fe, but the fix is the same - get more S in there and make sure the pH is right for S. This one is more tolerant of pH variations.
I think a lot of issues are pH-related, as said. Try to feed closely to your target pH when you see issues; often that alone will fix them up. Try not to correct pH with a flush unless you see deformation or twisting of the leaves. It's less shock if you can just subtly
persuade the pH back where it needs to go by feeding or watering it at exactly the pH you want it to have. Make sure you give enough liquid to cause some to drain out the bottom (but watch for those ones that just drain straight through - they usually have open channels, wasting the water. IMO that's why you should cultivate the top soil layer a little now and then - it fills those holes and then gets water to the whole root-ball). The pH will swing back usually without shocking the roots too badly.
Calibrate your pH meter if you haven't in a while.
I don't remember if you did this or not, but always feed immediately after a flush (unless it is the flush right before harvest). A flush tends to remove all the nutes, so you need to replace them or the plant will starve. Of course right before harvest starving the plant is kinda the point, so that's fine, but not before then.
Once again, I'm an amateur, I may be wrong, and I'm sure someone will (
please) correct me where i miss the mark. But that's my 2¢
