The Pool Calculator is correct as it would take 3.1 pounds of pure CYA to raise the level from 10 to 25 ppm in your 25,000 gallon pool. The pool store is wrong about not being able to use Dichlor to raise the CYA level. In fact, I did that when I opened my pool this season since my CYA level was too low! For every 10 ppm FC added by Dichlor, it also adds 9 ppm to CYA.
Every pound of Dichlor (dihydrate) will raise the FC by 2.7 ppm and the CYA by 2.4 ppm in your pool so you can certainly use it since you need to add chlorine anyway and it's a reasonably priced source of CYA accounting for the chlorine content as well (I compare prices here). The only thing I didn't like about the Dichlor was that it didn't dissolve as thoroughly as I thought it would, though that may depend on how granular or powdery it is so probably depends on the brand.
Richard
adding muriatic acid
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