Which chemical is commonly used to lower pH levels in pool water?

Hydrochloric acid is commonly used to lower pH levels in pool water because it is a strong acid that efficiently reduces the pH when added to water. When the pH levels in pool water are above the desired range (typically between 7.2 and 7.8), lowering it is essential not only for swimmer comfort but also for the efficacy of chlorine and other sanitizers.

Sodium bicarbonate is typically used to raise pH levels while also alkalinity. Calcium carbonate serves a similar function; it contributes to the overall alkalinity and can raise pH levels if the water is low. Algaecides are chemicals used to control algae growth and do not serve the purpose of adjusting pH levels directly. Therefore, hydrochloric acid stands out as the correct choice for effectively lowering pH in pool water.