There used to be, somewhere on the forum, some ramblings/musings/thoughts about dry acid and plaster pools. The thoughts IIRC, were that dry acid (which forms sulfuric acid when mixed with water) might not be the best thing for plaster/concrete chemically.

Muriatic acid (hydrochloric acid) chemically is a little 'nicer' to plaster/concrete. And it's usually less expensive too. But it is a bit more dangerous to use.

And ALWAYS remember to add acid to the water, never-never-never add water to the acid.

I'll usually add muriatic acid to a dedicated, clean 5 gallon plastic bucket filled with water. And then add the partialy diluted acid into the pool deep end. Rinse the bucket in the pool water in the low end, and put away till next time. Watch out for the acid fumes, wear chemical resistant gloves, be careful.