Mike-- Welcome to AFN mate!
....I need some more info,.. if you look in the highlighted section above this one, that's what we call a "sticky" thread, a reference thread basically,... in there you'll see a Patient Form to copy and paste into here, and fill out what you can,... secondly, indeed HID/LED light is horrible for diagnostics, so please post normal light pics when you can,.. color, texture etc. are important clues sometimes-
..also, unless this is a trick high end HPS bulb with better than usual blue spectrum output, it's suboptimal for veg'ing, and makes the CO2 pointless,... CO2 is really only effective when all other limiting factors are taken out of the equation-- light, nutes, temp/RH%.... just having it around burning away doesn't automatically equate to better, bigger plants,...
...you're in coco, right? Autopots--
,... have you read up on coco's odd characteristics concerning CEC (cation exchange capacity)? Being in Autopots, a lot of the do's/don't in coco are off the table, but Ca-Mg is critical in coco,.. it has a powerful attraction to surface binding of Ca, Mg to a lesser extent, and will hog it, keeping unavailable to the plant until it's sated, then it gives/takes readily like it should,... it's aform of buffering, not pH in this case, but with the exchange of cations,.. coco has elevated K levels, and in it's raw state (unprocessed/aged) a ton of K and Na (toxic at low levels),... this is why it's important to use reputable coco brands! Undercooked, badly washed and buffered coco will be an instant disaster! You don't have any problems related to this so far as I can see, so that's all just a heads up,...
...Hmmm, from what I can tell, and given that the other plants are OK, I'd say you might have a mutie girl here, something inherent in the seed, not grow related,.. but pics will confirm,... shots of the OK plants will help with comparison,...
