Stop spraying your plants with just water. That can leach nutrients from the leaves. You should do a foliar feed with a complete fertilizer mixed half strength and adjusted to pH 6-6.5. There seem to be a couple things going on there. Ca deficiency may very well be one of them. As Weedhound said, you will need to add calmag plus to distilled water. Your high tap pH is quite likely CaCO3 in the water; plants need Ca and the carbonate ion acts as a buffer. I've seen growers use distilled water to 'cut' their hard tap water and bring it to a useable level. But you need a EC meter to do that consistently.
Do you know what the temperature in the res is? Is it sitting on something cold?