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I just run the pumps on high, take the dry acid and slowly pour it into the water that is moving good from the jets.. as soon as the dry acid hits the water it is getting all sorts of tossed around and I'm sure disolves pretty quickly. I then let the pumps run on high for a full cycle (20 minutes) before they automatically shut off.
I've added plenty of Ph decreaser directly to the spa with pumps running- no apparent ill affects. I now use Muriatic acid instead of Ph decreaser, and I do exactly as Chas said. Take a plastic bucket, scoop in some spa water, add the acid, swirl to mix, then gradually introduce to spa water, pumps running. No problems, just have to handle the muratic very carefully- it's pretty strong.