Regarding reducing pH/alkalinity:
The *most effective* (not to be confused with *only*) way to do it is to add your reducing agent (muriatic acid, sodium bisulfate, etc) while the tub is off and the water is calm. Running the jets is fine. But if the water is calm, you get more bang for your buck, and the results occur faster.
Tip: If you're filling your tub after a drain, and you typically add, say, 5 cups of muriatic acid, dilute the 5 cups in a gallon of water, pour it into your footwell and begin filling your tub. Buy the time the tub is filled and the water circulates for a few mins the chemistry should be where you need it to be.
Good luck moving forward.