What is a good alternative to muriatic acid for lowering pool pH?

Use sodium bisulfate (dry acid). It’s safer to handle than muriatic acid, less aggressive, and available as a granular powder at any pool store. Start with about 1 pound per 10,000 gallons to lower pH by 0.1–0.2, then retest. Avoid vinegar or lemon juice—they require too much volume and can feed algae.