How are you measuring pH? Digital meters will lie at some point. They don’t last long even with proper care. 400 ppm is fairly hard but there’s much worse out there. You may want to filter it a bit to get it under 300. It will be difficult to maintain an acidic pH with high calcium (alkaline) water. Cheap pH down would mainly consist of…

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A seedling is in the vegetative state as soon as it’s born. It takes about a month for a seedling to become sexually mature in any situation. Doesn’t mean it’s going to ever show it’s sex in veg. Seedlings are initially provided nutrition through their embryonic sac, so they don’t need nutrients for about the first 10-21 days depending on the cultivar.

No such thing as a cal-mag deficiency. Calcium and magnesium are two completely different elements with completely different deficiency symptoms. Your plant is lacking neither. This random splotching is typically from pH swinging quickly out of range. Leaf twisting often occurs as well.