Phosphates are a pH buffer like carbonates and borates (and cyanurates). And the nice thing about phosphate buffers is that they are most effective (or have the highest buffering capacity) at a pH of 7.4 which is perfect for recreational waters. However, there is always a downside...phosphates will react strongly with calcium to form an insoluble precipitate (calcium phosphate and calcium hydroxyapatite ....the same stuff your teeth are made of) that is immune to removal even from the strongest of acids. So, one can use phosphate buffers in a spa, but realistically the CH has to be very close to zero or else you could get scaling all inside your plumbing, the heater especially and your filter. It may be possible to add the buffer to the water, create a bit of temporary cloudiness (scale) and then have the filter remove most of it but that's taking a risk. I certainly would not do that with the heater running as the heater will be most susceptible to scale. If you can fill from a water softener that reduces the CH to zero, then you could use that product to help balance pH.