Blue dream, not a shrimp

I think your original idea of Calcium was accurate. The damage will not heal it will just stop progressing on new leaves. The old leaves where it already showed damage is likely just necrotising a bit and I think this is normal for that leaf. It should not be appearing on any other new leaves though. If it is, this might be a build up issue. I do think the feed of 1.5EC is a bit high for the plants size, and I hope you are getting some runoff in the feeding/watering process. Double check their suggested rates of what bottle and the amounts then dilute down to a ratio you think is best. I feel 1.2 is ok for the size of this plant. if you can check the runoff EC and PH that may help show whats up, not the first bit that comes out but the mid/end stream. Good Luck!