Magnesium deficiency?

3-4 weeks old? A chance you cannot see a Mg-deficiency at this point.. if diary age is accurate. It takes 4-6 weeks to see visible symptoms of an Mg issue after it starts. coco is no different than any other soilless substrate. The only time it needs more of Ca or anythign else is because it was manufactured poorly or it was 'raw' and you were expected to buffer and balance it out to make it safe for plants... Assuming a typical 1.3-1.5EC concenetration of fertilizer (not including what is in your water to start, just fertilizer), you want 50-80ppm of Mg. Hard water areas may need a little less. if you see you are giving sufficient Mg, i'd start considering any of the other number of things that can cause interveinal chlorosis (a leaf chart will help). I think the color is at least 1-part optical illusion due to the rippels in the leaves. it's swollen and you get that darker look in the crevices... take the plant out into normal lighting and it may look a solid color. i see tip / serated tip burns.. i don't think this is Mg-related. P tox, K-tox and K-deficiency are possible, but the deficiency would come with interveinal chlorosis, which i'm not certain i see here until you make sure the leaf color is not an illusion of the grow light and a swollen (full of water, probably just genetics) leaf surface. 40-60 ppm P 180-200 ppm K at typical 1.3-1.5EC fertilizer concentrations. if drastically adifferent, could be the cause. With soilless/hydro you control everything in regard to fertilizer... so know what you add. it'll make diagnosing easier with fewer false guesses.. because relying on just visible symptoms leads to mistakes.. causality is not discrete here. Many symptoms have multiple causes, and at which point you need to cross reference other info to rule things out.... or you simlpy have to guess, which is a self-inflicted problem in the context of soilless/hydro.