Does your CYA test get even a little cloudy even though it doesn't obscure the black dot when the tube is completely filled? Are you mixing half (7 ml) pool water with half (7 ml) CYA reagent (melamine, actually) for at least 30 seconds?

You probably have at least some CYA in the water or else your chlorine would get used up very, very quickly. I just don't understand why it's not getting measured.

A saturation index of -0.2 is no problem at all.

As for pH and concrete, low pH is actually used during some processes (acid wash and initial curing) of concrete but after concrete is cured and stable it should not be exposed to very acidic conditions though it really has more to do with not having calcium carbonate saturation. Essentially with corrosive conditions (low pH, low TA, low CH) you dissolve the plaster/gunite/grout into the water, albeit slowly.

Richard