Pool chemicals explained

Updated: July 30, 2026

The pool aisle sells about thirty products that do roughly eight jobs. Here is what each chemical actually is, when you genuinely need it, and what it quietly does to your other readings — because almost every product fixes one number and moves another.

The eight jobs, and nothing else

Sanitize the water, adjust pH, adjust alkalinity, protect chlorine from sunlight, keep calcium in range, keep a salt cell fed, clear cloudy water, and kill algae. Everything on the shelf maps to one of those. Products marketed as “conditioner”, “stabilizer”, and “CYA” are the same chemical; so are “pH decreaser”, “dry acid”, and “sodium bisulfate”. Knowing the real name is the fastest way to stop overpaying for a relabelled bucket.

Sanitizers: what actually kills things

Liquid chlorine (sodium hypochlorite, 10–12.5%) is the workhorse: it adds only chlorine and a trace of salt, so nothing accumulates. Cal-hypo is granular and strong but adds calcium hardness with every scoop, which matters in hard-water areas. Dichlor and trichlor tablets are convenient and add cyanuric acid every time — this is the single most common reason a pool's CYA climbs all season until chlorine stops working. Bromine is the usual spa alternative, stable in hot water but not sun-protected.

pH and alkalinity: the pair that moves together

Muriatic acid lowers pH and alkalinity at once, cheaply. Dry acid does the same job in granular form with less fume, at the cost of sulfates building up over years. Soda ash raises pH strongly and alkalinity moderately. Baking soda is the mirror image: alkalinity strongly, pH barely. Borax raises pH with less alkalinity movement than soda ash. Pick based on which of the two readings is actually wrong — that choice is the whole skill.

The three you cannot undo

Cyanuric acid, calcium hardness, and salt only ever go up. No chemical removes them; the only route down is replacing water. That asymmetry should change how you dose: aim for the low end of the range and top up, rather than aiming high and discovering you have to drain a third of the pool. It is also the reason trichlor tablets deserve suspicion — they raise CYA invisibly, one convenient puck at a time.

Clarifiers, flocculants and algaecides

Clarifier clumps fine particles so the filter can catch them — slow, gentle, and the right first try for haze. Flocculant drops everything to the floor to be vacuumed to waste; faster but it bypasses the filter and wastes water. Algaecide is mostly a preventative: once you have a visible bloom, chlorine at shock level does the killing and algaecide is an expensive supporting act. Copper-based algaecides can stain surfaces and turn blond hair green.

The combinations that are genuinely dangerous

Never mix concentrated chlorine and acid — the reaction releases chlorine gas and it does not take much. Never mix cal-hypo with trichlor, in a bucket or a floater or a skimmer; the combination can ignite. Never pour water into acid, always acid into water. Store oxidizers and acids apart, in original containers, off the floor. If you only remember one rule: add one product at a time, into the pool, with the pump running.

Which product for which reading

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