Pool and spas are intentionally over-carbonated in order to provide a pH buffer and to saturate the water with calcium carbonate to protect plaster surfaces. However, such over-carbonation means that carbon dioxide can outgas resulting in a rise in pH. The amount of over-carbonation is significant. This chart shows the degree of extra carbon dioxide in the water compared to the normal amount when in equilibrium with air.
The outgassing of carbon dioxide occurs more quickly when the pH is lower and when the TA is higher (as more of the carbonates are in the form of carbon dioxide in these two cases), when there is more aeration (faster outgassing with greater air/water surface area), and at higher temperature (all reactions are faster and equilibrium is shifted more towards air).
When using net acidic sources of chlorine (including Dichlor which is really net acidic since chlorine usage/consumption is acidic), a higher TA not only provides pH buffering, but helps keep the pH stable because the carbon dioxide outgassing has the pH rise which compensates for the lowering of pH from the chlorine. When in balance, the pH remains the same, but the TA drops over time. In practice when using Trichlor which is very acidic, one uses pH Up which both increases pH and TA. Most pH Up products are sodium carbonate which is the equivalent to a combination of baking soda (sodium bicarbonate) with lye (sodium hydroxide).
When using hypochlorite sources of chlorine, one needs to have the TA be lower to get pH stability. If the pH tends to rise over time with this source of chlorine and it isn't due to plaster curing or other factors, then the TA should be lowered, not raised, in order to get a more stable pH. The recommendation for a TA of 80-100 ppm for hypochlorite sources of chlorine is not always low enough, especially when there is more aeration. Saltwater chlorine generator (SWG) pools in particular have a tendency for a greater rise in pH so could operate with lower TA levels of 70 or even lower, along with a higher pH target of around 7.7. If a lower TA is used, then the Calcium Hardness (CH) can be raised to keep the saturation index near zero.
In spas, there is even more aeration and the temperature is higher, so using a hypochlorite source of chlorine (such as with the Dichlor-then-bleach method) requires an even lower TA, usually around 50 ppm. For additional pH buffering, 50 ppm borates can be used as found in products such as Proteam Gentle Spa. The easiest way to add the borates is to use boric acid since it is only slightly acidic (one can also use a combination of 20 Mule Team Borax and acid added alternately, but that's trickier to do in small spa volumes).
The equilibrium pH when outgassing would stop is the following at 85ºF temperature and assuming no CYA or borates so that TA is equal to carbonate alkalinity.
TA ..... Equilibrium pH
120 .......... 8.6
100 .......... 8.5
80 ............ 8.4
60 ............ 8.3
50 ............ 8.2
40 ............ 8.1
30 ............ 8.0
20 ............ 7.8
10 ............ 7.5
Fortunately, it is not necessary to get to a TA of 10 ppm when using hypochlorite sources of chlorine. The outgassing rate is apparently proportional to the square of the TA level so the rate drops dramatically as the TA is lowered. Usually in most pools using a hypochlorite source of chlorine, the pH rises slow enough to be tolerable when the TA is 70-80 ppm though some pools with a lot of aeration may need a TA as low as 50 ppm. Spas have much more aeration and are far more likely to need a TA near 50 ppm (plus the use of borates as described earlier).