Pool chemicals explained
Updated:
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 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 hot tub alternative. The MAHC Annex notes it holds its strength better than chlorine at the higher pH common in spas, but sunlight destroys it quickly and CYA cannot stabilize it.
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. The MAHC Annex notes that copper algaecides can stain if pH is not controlled, and that simple quat algaecides foam when overdosed while polyquats do not.
The combinations that are genuinely dangerous
Never mix pool chemicals with each other. Concentrated chlorine and acid release chlorine gas. An EPA-accepted trichlor label warns never to add it to a feeder holding remnants of any other product, because the mix can react violently and cause a fire or explosion, so cal-hypo and trichlor must never share a bucket, floater or skimmer. Never pour water into acid; always add acid to water. The CDC advises storing each chemical separately in its original labeled container, keeping incompatible chemicals apart, and storing them below 95°F out of direct sunlight. If you only remember one rule: add one product at a time, into the pool, with the pump running.
Which product for which reading
| Reading to fix | Product | Also known as | Watch out for |
|---|---|---|---|
| Free chlorine low | Liquid chlorine | Sodium hypochlorite, bleach | Weakens in storage; keep it cool |
| Free chlorine low (granular) | Cal-hypo | Calcium hypochlorite, shock | Adds calcium hardness |
| Free chlorine low (tablets) | Trichlor | Pucks, tabs | Adds CYA and is acidic |
| pH high | Muriatic acid | Hydrochloric acid, pH decreaser | Lowers alkalinity too; fumes |
| pH high (granular) | Dry acid | Sodium bisulfate | Adds sulfates over time |
| pH low | Soda ash | Sodium carbonate, pH increaser | Raises alkalinity too |
| Alkalinity low | Baking soda | Sodium bicarbonate, alkalinity increaser | Barely moves pH, by design |
| CYA low | Stabilizer | Conditioner, cyanuric acid | Dissolves slowly; cannot be removed |
| Calcium low | Calcium chloride | Hardness increaser | Exothermic, pre-dissolve |
| Salt low | Pool salt | Sodium chloride | Use non-iodized, no anti-caking agents |
FAQ
What pool chemicals do I actually need to start?
For most chlorine pools: liquid chlorine, muriatic acid, baking soda, and stabilizer. That covers sanitizer, pH, alkalinity, and sun protection. Add calcium chloride only if your fill water is soft, and salt only if you have a generator.
Is pool shock the same as chlorine?
Usually yes, most “shock” is cal-hypo or dichlor, which is simply chlorine in a stronger, granular form. Shocking is a dose level, not a distinct chemical: it means raising free chlorine to roughly 40% of your CYA and holding it there.
Can I use household bleach in my pool?
Yes, plain unscented bleach is the same sodium hypochlorite as pool-grade liquid chlorine, just weaker, 8.25% versus 12.5%, so you need about 1.4 times as much. Avoid anything scented, thickened, or splash-less.
What happens if I mix pool chemicals?
At best you waste them; at worst you get a fire or toxic gas. Acid plus chlorine releases chlorine gas, and an EPA-accepted trichlor label warns that contact with any other product can cause a fire or explosion. Always add one product at a time, directly into the pool, with the pump running.
Why does my CYA keep going up?
Almost certainly trichlor or dichlor, both of which add cyanuric acid with every dose; dichlor adds about 0.9 ppm of CYA for each 1 ppm of chlorine. Switch routine chlorination to liquid chlorine or cal-hypo, which add none.
Sources
- CDC, Model Aquatic Health Code, 5th edition (2024), Annex. Sections 5.7.3.1.2.2 (bromine, pH and sunlight), 5.7.3.1.3.1 (trichlor and dichlor add CYA) and 5.7.4.4.4 (algaecides)
- CDC, Pool Chemical Safety (June 24, 2025). Store chemicals separately, in original containers, below 95°F
- U.S. EPA, accepted label for Trichloroisocyanuric Acid (TCCA) 90, EPA Reg. No. 103874-1 (April 9, 2025). Trichlor can react violently with remnants of other products
- Trouble Free Pool, CYA Chlorine Relationship (Chlorine/CYA chart). Shock level as a share of CYA
Related
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