Too big for nute burn if you only gave a bit more than usual, it seems the temps are too high and venting poor.
If it's well water, calcium should not be an issue. Sodium, now ... do you know if your water has sodium? Are you perchance using a water softener? Those'll put sodium into your water, which can lock out K, Mg, Ca (esp. since it's been removed).
As for the greenhouse ... definitely open it up during the hot. That could be a big part of it. cn