Hot tub total alkalinity, explained

It is the water's buffer. Alkalinity measures the carbonates and bicarbonates that soak up acid and base, so it decides how hard pH is to move. With low alkalinity pH jumps around with every chemical you add; with high alkalinity pH barely moves no matter what you add.

Fix alkalinity first, then pH. Bullfrog's manual says to bring alkalinity into range before adjusting pH even if it throws pH off further, then correct pH once alkalinity holds. Hot Spring makes the same call and says to disregard pH readings while alkalinity is being adjusted.

Raise it with a spa alkalinity increaser, which is sodium bicarbonate; lower it with the same sodium bisulfate product that lowers pH. Hot Spring names both. Add a little, run the jets, wait the time your manual gives and retest. Once balanced, Hot Spring says alkalinity normally holds until the next refill, and suggests checking it once a month.