So when I opened the pool back in May, my TA was low (50 or 60). I raised it with baking soda to 70. I've been doing straight BBB since then, and I've noticed the TA never changes from 70. I've probably used 5 lbs or more of dry acid, which should have lowered it by 12. In addition, I have a lot of aeration from a fountain and have backwashed quite a bit (all of which should have lowered it even more). Is this an expected behavior? How are these carbonates/bicarbonates magically getting replenished in my pool?