Jason is right. I believe I was responding to someone who wanted a chemical that JUST lowered TA without lowering pH and there isn't any (see
this thread where I made my post -- the question being answered was "Shouldn't there be a jug of TA down that would make this process much faster?"). When I said that aeration is what is used to lower TA, I should have been more clear and said that it is the COMBINATION of aeration with acid addition that lowers TA. As Jason said, aeration raises pH with no change in TA while acid addition lowers both pH and TA. The net of aeration+acid is a lowering of TA.
The
Lowering Total Alkalinity in the Pool School describes the procedure. As Jason pointed out, having borates already in the pool makes it more difficult to move the pH. The TA still gets lowered by the same cumulative amount of acid, however. In a 15,000 gallon pool, 38-1/2 fluid ounces (almost 5 cups) of full-strength Muriatic Acid (31.45% Hydrochloric Acid) will lower the TA by 10 ppm. If you have borates in the water, then the pH won't drop quite as much when you add the acid.
[EDIT] Larry at another pool forum put a table I wrote into a nice format that explains the procedure and also shows the effects on pH and TA of each step
here. [END-EDIT]
I'm not sure why your TA went from 70 ppm to 90 ppm after you added borates and muriatic acid. The TA should only have gone up by 5.2 ppm if you added 50 ppm Borates and the pH before and after was the same.
[EDIT] By the way,
if you are having that much trouble getting the pH to rise even with aeration, then you may not need to lower the TA in the first place. The only reason for the lower TA recommendation with SWG pools is that they tend to rise in pH, at least partly from carbon dioxide outgassing. If in your pool the pH is already pretty stable at the current TA level, then you can just leave it as is. Perhaps over time the pH will slowly rise and you can then add some acid to lower it and the TA will go down somewhat when you do that. In the meantime, if you want to raise the pH to 7.5 right now and can't wait for aeration to do so, you can use about 8 cups of Borax. Just keep in mind that the TA will probably rise to around 100 ppm if you use Borax instead of aerating, but this is probably not a big deal in your situation. [END-EDIT]
Richard