Your salt mix will bring your pH up to the desired level (about 8.2) if you use
RO water. pH may also change as it mixes in your container. That's why you let the water mix up for about 24 hours before you use it as your changing water, so the pH and also the salinity are stable. I have no choice but to use
RO water for my salt tank; my tap water is so bad that I use some
RO water now for my
FW tanks. If you're using tap water, it should be free of phosphates (phosphates can feed algae). Test the phosphate level of your tap water, and try mixing up some salt mix and see what the pH ends up. I think
CC will also boost pH, but be careful, don't just throw some in there without testing your pH fist. Also, steve-s said that
CC can be a detrius trap, and sand is an easier substrate to work with. (I may have a very small amount of
CC because it seems that the fish I want likes to play in it!)
Anyway, I would try mixing some salt mix with your tap water and then check your pH, and see how stable it is too.