How much acid can I safely add at once?

[ragzilla]

In about 2 weeks we should have our 375 gallon hot tub finally in place, and on the advice of this this forum and r/hottub I went ahead and got the Taylor test kit and just tested my fill water last night, and learned some more things about the wonderful limestone aquifers the ground source water here in Indiana comes from.

7.6 pH | 350ppm TA | 475ppm CH (unsoftened)
310ppm TA | 50ppm CH (softened, skipped pH, hot side faucet if that changes much)

To deal with the CH, I'm planning to have a plumber run a new outside spigot off the hot water (continuous, gas) heater which is downstream of the softener, I have an external bypass on the softener so I can mix it appx 35/65 hard/soft to get it down into around 200ppm (and as a plus, I can do the initial fill with the water heater dialed down to 100f so it won't take forever to heat up), but I'm struggling with how quickly I can bring the TA down so I can get the rest of the chemistry done.

PoolMath says I need 27 fl. oz of 31.45% muriatic, or 36 oz of dry acid to drop TA ~280ppm, but with a 7.6pH I'm only seeing I can add 1oz of dry acid, or 0.75 fl. oz of muriatic before the pH will go below 7. Or should I pay more attention to the TA being consumed, as that carbonate is currently buffering the pH?

 

[Texas Splash]

Focus more on adjusting the pH. That will take care fo the TA as well. So allow the pH to rise to about 8.0 then use just enough muriatic acid to lower the pH down to about 7.0. The pH will eventually rise back up and then you repeat the process. Each time you do that you should be lowering the TA as well.

 

[Methuselah]

Evaporation will require periodically adding more water, so some of your CH number will drop to splash out, but not as much as you will add. Target accordingly. Also note some CH is good, excessive can precipitate and also coat your heater element, so a read on saturation index might be worthwhile as well.