Muriatic acid is the best alternative to pool pH decreaser.

Yes, you can use it — just be careful.

Commercial pH decreaser is usually just sodium bisulfate (dry acid), repackaged and marked up. Muriatic acid (hydrochloric acid) is cheaper, more concentrated, and does the same job faster. You use way less of it, which means fewer trips to the pool store.

Downside: it’s nasty stuff. Wear gloves and goggles, add it slowly to a bucket of pool water first (never pour water into acid), and don’t splash. It’ll lower pH fast, so start with half the recommended dose and retest after an hour.

If you want to go totally chemical-free, CO2 injection works too — you bubble CO2 into the water and it forms carbonic acid.