Remember that the source water (tap water or trucked in) for filling your spa, hot tub or pool can have certain properties that can cause chemical changes as you top off your spa; especially calcium hardness & metals & minerals.
Even if using trucked in water, have a proper chemical analysis done of that to determine what & how much balancing needs to be done. Unfortunately, many believe believe that the "pool water" that is trucked in has been treated. That just isn't so. Trucked in water can be from a variety of sources including wells, fire hydrants, ponds, streams, etc.
Before adding chlorine, bromine or any oxidizer, have that water professionally tested. You'll save yourself time, money & potentially a lot of aggravation.