Lowering Alk

Unless you add baking soda to your pool (or "pH Increaser" that happens to be soda ash or sodium carbonate), then your TA will never increase. It will slowly decrease as carbon dioxide leaves your pool, but it may do so very, very slowly once you get down to low TA levels (less than 80) and maintain the pH high enough (7.4 or higher), but the exact levels and rates are dependent on the amount of aeration you have -- higher aeration means faster outgassing of CO2 and faster pH rise and lowering of TA (though the TA measures a drop much more slowly than the pH rises, all else equal).

This chart will give you some idea, but even my color coding of this chart is quite approximate and dependent on aeration. Nearly everyone experiences a noticeable pH rise (with TA drop over time) when in the red region and hardly anyone experiences one when in the green region, but there are exceptions. Also, the pH can change for other reasons so it's really watching the TA over time that tells you what's really going on.

And yes, your spa is a good source of aeration so you will likely continue to experience some pH rise and slow TA drop. You can either live with that (and continue to add acid and eventually some baking soda) or you can let your TA drop even further (probably not below 50 in any event). You generally don't need much pH buffering if you are using bleach -- it's Tri-Chlor tablets that really need the 120 TA. Normally a spa would also need substantial pH buffering due to the small water volume and relatively high bather load so for the spa, at least, you'll probably have to live with adding acid more regularly (but at least it's not very much, given the low volume of water).

Richard