When you add acid to the water, it lowers the pH and TA. Carbon dioxide outgassing then raises the pH with no change in TA and this occurs more quickly when there is more aeration and at lower pH. Basically what you've been doing is the "lowering TA procedure". You shouldn't have your TA lowered that much. There are two possible problems that exacerbated the situation: 1) you are lowering the pH by too much -- you should let the pH get to 7.8 if it wants to and do not lower the pH unless the pH gets higher than that (that is, do not try and maintain a 7.5 pH) and 2) you should be using 50 ppm Borates (usually from boric acid) as an additional pH buffer if you are not already doing so.
At this point, add a small amount of baking soda to get your TA up to 50 ppm. Also, add 50 ppm Borates if you don't have that already. And finally, don't sweat it if the pH gets above 7.5 -- only deal with it if it gets above 7.8.
Also, in
this recent post of yours, it indicates that you have no CYA in the water. Why is that? Have you not been using Dichlor to initially raise the CYA level? Are you using the Dichlor-then-bleach method described in the Pool School article
Using Chlorine In a Spa? Even when you get an SWG, you will still want CYA in the water. Without any CYA in the water, the chlorine will be too strong, will react too quickly with skin, swimsuits and spa materials, will outgas too quickly and react with the hot tub cover, etc. The post did indicate that you were going to get some and you can get pure CYA if you want to, but you can also use Dichlor to add both chlorine and CYA fairly economically and in small quantities (it may be hard for you to find CYA in small quantities for a spa).
