Low alkalinity and low calcium hardness in pebble tec pool

I have recently become a cpo, and am very new to balancing plaster pool water chemistry (previously only vinyl pools testing chlorine and pH). I'm seeking info on the best way to balance and raise both alkalinity and calcium hardness. Both have lowered progressively in recent weeks, while both ph and chlorine levels have remained normal. I need to get the calcium up, as I do not want to damage the pebble tec and I am unsure how long it can go unbalanced before it causes etching.

Thanks.
Jennifer

Pool volume 23,000
alkalinity 40 ppm
calcium hardness 110 ppm
pH 7.4