necrosis spots on leaves

Iv looked through your diary. Your environment seems fine from your figures, your Ec seems fine but maybe a little low for the size of the plant, your input ph is fine. Your using airpots which are great. The only thing I see is biobizz nutes, more and more people seem to be having problems and they all use that brand of nutes. I'm going to go out on a whim and say its bad watering practice that's causing your issues. Wait for at least the top inch of soil to dry before next watering, don't use nutes and just ph your water to 6.2 and make sure you get a good 20% runoff. Then wait for top inch to dry again and water with your nutes but aim for an ec1.3, and alternate between ph water and nutes. Soil likes to dry out between watering unlike coco.

This is not a calcium deficiency. You give the right type and the right amount of nutrients. You also give the right pH. So what happened here is that the nutrients waste salts builded up inside your soil. To prevent this you need to give water till it runsoff from your pot. You want 10% from the given water to runoff every time you water. But for the first time I would give 2 liters with your nutrient solution and let it drain good to prevent it getting worse!

Maybe Calcium, but the leaves going crispy and stating green is reminiscent of root issues, is your medium draining well? is your PH on point in runoff? I'd check these few things first and adjust if they are off. Also calibrate your tools if you have not in a while. I myself do not like 20/4 lighting schedule and feel plants need a bit more down time. 18/6 is my ideal. Best Of Luck!

May be a Calcium Deficiency