I have an overflow/waterfall in my pool that runs every day. I also try to maintain a lower PH ( 7.2-7.4 ) to help with an ongoing fight against metal stains. This combination of aeration and acid addition causes my Alkalinity to drift downward to 50 without intervention. When this happens, I add enough baking soda to bring it back up to between 70 and 80. I'm not really sure why I am doing this. It's tempting to let the TA stay at 50 because I know the acid demand increases after raising it. Can someone please explain why I want my TA to stay above 70?