Hello, my ph and alkalinity are running low lately. i have a dark bottom plaster pool, direct sun all day. I wasn't getting a high chlorine reading( but we have the nature 2 cartridge, so i think that's okay for it to be low) I shock every 2 weeks with at least 1 and 1/2 pounds( 15,000 gall) my ph is like 6.8, my alkalinity is about 90-100. The pool guy threw 10 pounds of baking soda in, cause he said you have to adjust alkalinity first. Levels are now what I just posted, but that is still low- what should I be, I'm thinking 7.2 for ph, and alkalinity 120?? Any advice? could the massive sun be messing with it?? thanks in advance, michelle