The problem with lowering Total Alkalinity (TA) in general is that when you add acid to lower TA, pH also decreases. This limits the amount you can lower TA without pH becoming too low. However, there is something you can do to raise pH that leaves TA alone, Aeration. Aeration is the process of turning on all the jets and air features in your tub, in order to pump as much air into the water as possible. By adding Acid and Aerating you can lower TA, while keeping pH the same. I won't get into detail why you would want to do this here, but I will say this. The key to a balanced tub is, you should NOT try to adjust pH directly. Instead, you should focus on Tuning TA to an ideal level, in order to keep pH in an ideal range.
This method is a slight variation to the Acid/Aeration method. The main problem with the old method is that it's difficult to know how much acid to add each time. The amount of acid it takes to lower pH to 7.0 will vary a lot depending on the TA. Also, if you have a very high TA (>300 ppm), it can take a long time to lower it if only small amounts of acid are added. So, I decided to refine the method a little. If done correctly, you can lower TA quickly, without pH ever dropping below 7.0.
The first thing to remember is, the amount that TA decreases is directly proportional to the amount of acid you add. That is, in a 350 gal tub, 8 oz of Dry Acid will lower TA by 100ppm. It is important to know (by calculating) how much Total Acid you need to add to your tub in order to get to your Target TA. The second (key) thing to remember is, the higher your TA, the less effect acid will have on your pH. If your TA is really high (>300), adding acid won't effect pH much at all. This confuses beginners (and some experts alike), which causes them to take much longer to lower TA, or give up altogether. Because of this key point, you need to add more acid in the beginning, and less as you get closer to your Target TA.
So, here is the process.
1. Turn on all your jets and air features and keep them on the entire process. (i.e. Aerate)
2. Check starting TA/pH.
3. If pH is < 7.6, goto step 6.
4. Calculate how much you want to lower your TA, and how much acid you need to add to lower it.
5. Add to the tub HALF (not > 1 cup nor < 1 TBS) of the amount of the remaining acid you need to add.
6. Aerate for 30min, and check TA/pH.
7 If TA > Target and pH is > 7.8, goto step 4
8 If TA > Target and pH is < 7.6, goto step 5 one time.
9 If TA > Target and you can't get pH > 7.8 via aeration, Target too low. Stop here.
10 If TA = Target and pH is > 7.8, Target too high. Lower Target TA and goto step 4.
Your TA is 300ppm, you want to lower it to 60ppm, in a 350 gal tub.
To lower TA by 240 ppm you need to add a total of 2.4 cups (~19oz) Dry Acid.
Turn jets/air on.
Add 1 cup acid and Aerate 30mins.
Check TA/pH. TA=200 and pH>8.0
Add 6 oz acid and Aerate 30mins.
Check TA/pH. TA=125 and pH>8.0
Add 3 oz acid and Aerate 30mins.
Check TA/pH. TA=90 and pH>8.0
Add 2 oz acid and Aerate 30mins
Check TA/pH. TA=65 and pH=7.8
Add 1 TBS acid and Aerate 30mins
Check TA/pH. TA=60 and pH=7.6
Again, the main thing to remember is, you add more acid in the beginning of the process, and taper off toward the end. This method will allow you to lower your TA in just a few hours, regardless of how high your TA is, without pH ever dropping too low.
Lastly, safety is priority one. Be careful with any type of acid, especially Muriatic. If you're using Muriatic acid, wear rubber gloves and eye protection. Muriatic Acid is nasty stuff. Dilute all acid in a bucket of water and slowly pour mixure over a return flow, or middle of tub. Be careful not to splash. Turn the Air off when adding if necassary. When mixing Acid and Water, always add Acid to water! NEVER ADD WATER TO ACID!!!