If by crystals you mean granular, then you want that over powder. The powder will not mix easily with the water, some will blow away in the wind and some will sit on the surface, so go with granular.
It dissolves pretty quickly, you can either slowly pour it in front of a return or broadcast it, brushing is never a bad idea after you do it. While it is a fairly weak acid you don't really want to leave anything sitting on the bottom of the pool too long.
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