Yellow leaves!

Usually that would come with spots,. Chlorosis is caused by more than one thing, so you need to cross-reference and eliminate possibilities. No spots rules out Mg deficiency, at least for now.. how it progresses is a big part of diagnosing the problem, too. Try not to get water on your leaves. Standing water is a disease vector. Foliar sprays are fine in reaction to a problem, but just doing so on a regular basis is probably a net-negative. Roots are teh best way to feed a plant, bar none. there should be no reason to constantly spray these plants. a bit mroe perlite would be good - want 33% give or take with coco. I don't think this is a bad batch of coco coir. The older growth looks fine. Rhetorical questions to ponder in the absence of enough information here... Did you increase light intensity recently? Even if 7-10 days ago... Are you using "hydro" nutes? These are nutes that are 100% plant ready and 100% plant-available nutes. They differ from soil nutes in this regard. Since you are using a soilless substrate you need to treat it as such. Fertigate every irrigation with a 1.3-1.5EC well-balanced mix of fertilizer (not counting your tap water EC). Better brands will be ph-buffered. This doesn't just mean ph-balanced, but also resilient to ph-drift, too. ph-balanced does not guaranteed a resiliency to ph-drift. this is why people think 'some' acids are more prone to drift but that's not really the case in the context of what is typically used. drift occurs because of poorly buffered solutiosn or bad environmental conditions. fertigate with 10% runoff, religiously. Wait for top layer to start to dry and repeat. don't deviate from that fertigation procedure. use hydrobuddy app or website to calculate ppms.. these are ballpark figures but the general ratio of nutes is rock solid. you may need a higher EC depending on your local VPD. 120-130 N 40-60 P 180ish K 100+ Ca 75ish Mg 100+ S -- Some of these may be supplemented by your tap water too - more likely the last three. If you have any nitrates in your water it'll usually be insignificant. A local water quality report, if available can help there, but simlpy observing and making minor adjustments to formula will be simple and easy - i.e. small, slow-moving issues, if any. ph around 6 is fine. Again, any fertilizer worth its salt for hydro/soilless should be ph balanced and buffered. you shouldn't have to mess with it, but if you do, acetic acid 5% (5% white distilled vineager) can help you ph-down for cheap.

I agree with GODAXE, but this looks like you are growing in coco and it might be a ph related issue...

i think any one will be agree that more information is need. plant genetique, age, nutrient brand and concentration, ph. just cant tell you if it will rain tomorow if i dont know where you are