For a quick overview of the process, see the Pool School article How To Lower Total Alkalinity
There are two reasons to lower your total alkalinity (TA) right away, because you want to slow down the rate that the PH rises, or if high TA is contributing to a high calcium saturation index (CSI) which puts you at risk of calcium scaling.
If you don't have one of these problems, there is no need to be in a hurry to lower your TA. The recommended TA ranges are fairly narrow, but most pools will be fine with somewhat higher TA levels. When you aren't in a hurry, you can lower TA by lowering PH to 7.2 every time it gets to 7.8. Over time that will have the same effect as the procedure below.Prerequisites
To lower TA:
- A good test kit that can measure PH and TA.
- A supply of acid, preferably muriatic acid, though dry acid/PH Down will do.
- A source of aeration.
- Patience. Lowering TA takes a while, often days or weeks.
- Optionally, an acid demand test can be useful.
- Lower your PH to between 6.8 (7.0 for vinyl pools) and 7.2 with acid.
- Aerate until the PH goes up to 7.6.
- Test TA, and repeat steps 1 and 2 until you reach the desired TA.
You can lower PH with muriatic acid or dry acid/PH Down. You can get an estimate of the amount of acid to use with The Pool Calculator. Keep in mind that the amount of acid required can vary from the estimate. If your test kit includes the acid demand test, you can use that to calculate the amount of acid required more precisely. Allow at least 30 minutes after adding acid, with the pump running, before testing the PH again.
Muriatic acid should be added by pouring it slowly in front of a return jet with the pump running. Dry acid should be pre-dissolved in a bucket of pool water and then poured slowly in front of a return jet with the pump running.
When lowering the PH, be careful not to go below 6.8 (7.0 for a vinyl pool). Keep in mind that the common PH test kit reads any PH level below 6.8 as if it was 6.8, so it isn't obvious just how low the PH realy is when it is 6.8 or lower.
Aeration can be provided by a SWG, spa jets, waterfall, fountain, return pointed up so it breaks the surface, air compressor, kids splashing, rain, etc. This post
and this one
show examples of home made fountains and this post
shows a spa style jet that fits into a return.
The more aeration you can provide, the faster the entire process will go. Many smaller bubbles are better than a few large bubbles. Lowering the PH back down more frequently will also speed up the process, though that can get tedious. Lowering TA can take some time, days or even weeks, depending on the amount of aeration you have and how far you need to lower your TA. Be patient.
Adding acid lowers both the PH and TA. Then aeration raises the PH without changing TA. If, instead, we added borax to raise the PH, it would also raise the TA back to where it started. Soda ash is even worse, since it will raise the TA higher than where it started.
It is actually the acid which lowers TA. Adding acid lowers both the PH and the TA. Then aeration raises the PH without changing TA. For a more detailed discussion of the whole process, see the Lowering TA sticky.
In a swimming pool, most of the TA comes from bicarbonates in the water. As we lower the PH, some of the bicarbonate is converted to carbonic acid. Carbonic acid is really just dissolved carbon dioxide, CO2, the same as is used in carbonated beverages. Aeration encourages the CO2 to outgas, so it can't turn back into bicarbonates when the PH goes back up.
There is a common myth that you can lower TA by adding "slugs" of acid to the pool. This does not work and can damage the pool surface. See this paper
for a detailed study showing that this approach does not work.