Popped up around last week (week 4, monday). Prior...

Hi @RunWithIt! This is Calcium-related (not Mag). This deficiency is known to appear when the pH-range is wrong. Soil has the ability to buffer your pH and roots in soil will always find a way to handle nutrient intakes (unless intake pH is really out of the range). But the coco medium doesn't have this ability and needs the solution's pH to be buffered upstream. You must apply the ideal pH range for coco, which is 5.5~5.9. Never exceed 6.0, otherwise you will encounter deficiencies and rootzone issues. Now, as mention by others, "hard water" defines a water that already contains a few nutrients (so it has its own basis EC), we're not talking about pH here, everybody needs to pH-down its tap water, but some will have a 0.1 basis EC while others can have a 0.6 EC without adding anything --> this is "hard water". If your tap water's EC > 0.3 then you probably don't need to add Cal-Mag (we suppose this high basis EC is because of different minerals, including Cal&Mag, but the only way to truly ensure this would be to see a laboratory test for your tap water). If your tap water's EC < 0.3, go for it and add a slight doses of Cal-Mag each time you water. Hope this will help, keep us up-to-date and happy growing 👊