Thanks on letting me know about the Ca I didn't remember and didn't look (did I mention lazy)i looked at that pic again. roughly how many leaves have the brownspots on them? i only see maybe 5 or so. are there any rust spots on any new growth?
and to answer your question: you could get 3 sources for Ca; calcium nitrate (ca, N) , calcium chloride or a bottle of cal/mg (N, ca, mg)
if there are less than 10 leaves with spots, i'm gonna guess your tap water is close to how much Ca you need edit: along with the DG bottles which have 2% Ca