Muriatic acid
Also called: hydrochloric acid, pool acid, pH down (liquid).
Muriatic acid is hydrochloric acid, the liquid pool owners add to lower pH. Every dose lowers total alkalinity at the same time, and no dose or pour technique drops one without the other.
Why it matters
Pool pH almost always drifts up, so a routine acid dose is normal. Broadcast it slowly over the deep end with the pump running, never as a concentrated slug in one spot, which etches plaster and doesn't spare your pH anyway. Which acid matters on a salt pool: dry acid leaves sulfate that only leaves by draining, Pentair's IntelliChlor manual says never to use dry acid to adjust pH, and Hayward's Aqua Rite manual discourages it. Muriatic leaves chloride, which the pool already has. If pH keeps climbing back, lower the alkalinity with acid, then aerate the pH back into range.
In numbers
A scaled salt cell gets a 4:1 bath of water to muriatic acid, acid added to the water and never the reverse, for 10–15 minutes until the fizzing stops.