Charlie,

Richard doesn't intend to be harsh on you but rather is upset with our on-going battle with pool store clerks who continually distribute mis-leading, false and even dangerous information.

Everything the clerk told you is totally wrong, which you would not have known--that's why you asked the question.

If you add acid to a pool without the filter running, you risk damaging it, if that "slug" of acid reaches the sides or bottom. This is particularly dangerous for vinyl pools as it can soften and weaken the vinyl where the water pressure is highest. You should ALWAYS add acid with the filter running and you should dilute it first. I like to pour it into a 5-gallon bucket of water first. Then it is far less likely to damage the pool (who cares if the wall of the bucket is etched, right?).

Pool stores have been teaching the "slug" method for lowering Total Alkalinity for years--it doesn't work. Funny, the clerk said it would NOT lower your alkalinity--he's right for the wrong reason and wrong for the right reason.

See, T/A AS MEASURED goes up as pH rises and down as it falls. You can raise T/A additionally by adding baking soda or washing soda (sold expensively as "Soda Ash" or "pH Up!" in pool stores, and cheaply as Arm&Hammer Washing Soda).

To lower T/A, you use the "trick" of lowering it by lowering the pH. But now you have to find a way to RAISE pH without raising T/A as well. The only practical way is by AERATION.

Aeration can be accomplished with a fountain, a gang of 12-year olds, a waterfall, or just leaving the pool uncovered with the returns pointed at the surface.

Since NONE of this is known to pool store clerks, they still recommend the old slug method. This sort of seems to work but only if:
a) pH is lowered sufficiently--but adding the acid with the filter running will do it faster.
b) Aeration happens unintentionally--by leaving the pool uncovered and having a bunch of kids in it.

As for Richard's other comments, they are WELL worth pursuing.
1) pH is known to rise with SWG pools. Lower T/A levels, like around 80-90ppm, should help, but you may still find you have to add more Muriatic Acid than anything else.
2) Borates at the 50ppm level should help stabilize the pool as well.

BTW, if your pH is over 8, you are already in trouble and need to get it into the 7's or the effectiveness of chlorine will be drastically reduced.