Sour Crack

Hey there Paul, sorry to disappoint you but you most likely don't have zinc deficiency! Such a deficiency is exceedingly rare in soil, mostly it's a "lab" thing, and in your case it's not what's affecting you. You have two issues: burnt tips and Mg deficiency. Burnt tips are from a (slight) excess of NPK fertiliser, most likely the nitrogen. The yellowing you're seeing well that's the Mg part. I see you're on a 24hrs light schedule. There's a very good reason you don't see any successful grows with 24hr light schedule: the plant goes into overdrive and needs more nutrients especially mg which is used to regulate photosynthesis when it's lights on, so even though you've been feeding Mg, well the plant just needs more. The plant's now stressed and the problem will start spreading eventually becoming pretty bad the leafs will burn on starting with the edges. For sure you'll need to reduce the light cycle and I suggest reducing it to 18/6 to reduce stress, but I know you'll end up reducing it to something less than that. Try also making a foliar spray with your calmag and spray the plany shortly before lights on. Increase it in your feed too. For the pH issue, I see you're pH ing down, hopefully using a store bought pH down and not some kind of homemade pH down recipe. Next feed cut out the algamic all together because it will raise your soils pH and it contains N in it which are two of your problems. Hope this helps ! 🚀