Pool Chemistry Basics: Every Parameter Explained

What pool chemistry actually does

Balanced water keeps your pool clean, clear, and safe to swim in. It also protects the pool shell, tile, plumbing, and heater from corrosion or scaling. Most pool problems trace back to one or two parameters being out of range, not to mysterious causes.

There is one critical idea to understand upfront: free chlorine (FC) and cyanuric acid (CYA) are linked. Higher CYA means you need higher FC to get the same sanitizing effectiveness. The old advice of 'keep chlorine at 1 to 3 ppm' is wrong for any pool with CYA above 30. PoolSense uses the FC/CYA ratio for every recommendation.

Free Chlorine (FC) — your pool's main disinfectant

Free chlorine is the sum of active chlorine (the part doing the work, killing bacteria and oxidizing waste) and reserve chlorine (the part bound to CYA, protected from sunlight). FC is your pool's defense against algae and pathogens. If it drops to zero, the pool is unsafe.

Target: based on your CYA. As a quick reference, CYA 30 → FC 2 to 4 ppm; CYA 50 → FC 4 to 6 ppm; CYA 70 → FC 5 to 9 ppm. Salt water pools run slightly lower FC for the same CYA because the cell makes chlorine continuously.

Test daily for manually dosed pools, every 2 to 3 days for salt or automated systems. If FC drops below the minimum for your CYA, follow the SLAM process (shock + sustain) to recover.

• To raise FC: add liquid chlorine, increase salt cell output, or run the pump longer
• To lower FC: wait — sunlight and organic load lower it naturally
• Avoid household bleach with 'splashless,' fragrances, or '4 in 1' additives. Use plain sodium hypochlorite

Combined Chlorine (CC) — the 'chlorine smell' indicator

Combined chlorine, also called chloramines, forms when chlorine reacts with sweat, urine, sunscreen, and other contaminants. CC is the chemical that makes a poorly-balanced indoor pool smell sharp. A healthy outdoor pool keeps CC at or near zero.

Target: below 0.5 ppm. Above 0.5 means your sanitizer is fighting a backlog and needs help. The fix is the SLAM process — shock to a sustained high FC for your CYA, brush, and run the pump 24 hours. It is not a one-time bag of shock; it is a 1 to 3 day commitment until CC is back below 0.5 and FC holds overnight.

Cyanuric Acid (CYA) — the chlorine stabilizer

CYA, also called stabilizer or conditioner, is sunscreen for chlorine. Without it, UV burns through your chlorine in hours. With too much, chlorine gets bound up and loses effectiveness even when the FC reading looks fine.

Target: 30 to 60 ppm for liquid chlorine pools, 60 to 90 for salt water pools (the cell needs higher CYA to be efficient). Above 90 ppm and chlorine becomes ineffective regardless of FC.

Trichlor tabs and dichlor shock both add CYA every time you use them. If you rely on trichlor tabs in a feeder, your CYA climbs steadily through the season and you eventually hit chlorine lock — high FC reading, but the pool still gets algae.

• To raise CYA: add granular stabilizer using the sock method in front of a return jet
• To lower CYA: partial drain and refill, or reverse osmosis water exchange — there is no chemical fix

pH — comfort and equipment protection

pH measures how acidic or basic your water is. Low pH (below 7.0) burns eyes and corrodes metal. High pH (above 8.0) clouds water and forms calcium scale on tile and inside salt cells. The good news: a wide range of pH is OK for swimming, and pH naturally drifts up over time in most pools, so frequent adjustment is normal.

Target: 7.2 to 7.8, ideal 7.5. PoolSense recommends muriatic acid to lower pH (slow pour, deep end, pump running). For raising pH from below 7.0, 20 Mule Team Borax is preferred — it raises pH without changing alkalinity much.

If pH keeps rising fast, suspect: high total alkalinity, frequent Cal-Hypo shock additions, or aeration from a salt cell or water feature. A solar cover slows pH drift.

Total Alkalinity (TA) — the pH buffer

TA stabilizes pH and prevents wild swings. Low TA leads to unstable pH. High TA causes pH to drift up fast, requiring frequent acid additions.

Target: 50 to 90 ppm for liquid chlorine and salt water pools (TFP guidance). 80 to 120 ppm is the older pool-store target that assumes trichlor tab use — trichlor is acidic so higher TA buffers it. If you do not use tabs, lower TA reduces how often you add acid.

• To raise TA: baking soda (sodium bicarbonate), in small increments, re-test every 30 minutes
• To lower TA: muriatic acid plus aeration. This is the slow part — over several days, in small acid doses with aeration after
• Pro tip: test your fill water TA. If your tap is 200+ ppm TA, you will fight high pH constantly

Calcium Hardness (CH) — protecting surfaces and equipment

CH is the calcium concentration in your water. The right level protects plaster, grout, tile, and the heat exchanger inside a gas heater.

Target: 250 to 650 ppm. Plaster pools and pools with heaters need at least 250 to prevent etching and corrosion. Above 650 ppm causes scale on tile and inside salt cells.

Vinyl liner and fiberglass pools do not strictly need calcium, but high CH still causes scaling. Test your fill water — some regions have CH above 200 in tap water, so even with no calcium added, CH climbs as water evaporates and is replaced.

• To raise CH: calcium chloride (also sold as 'hardness increaser')
• To lower CH: partial drain and refill, or reverse osmosis — no chemical fix

Salt — only for salt water pools

Salt water chlorine generators (SWGs) need salt to make chlorine. Salt level is not a sanitizer in itself; it is fuel for the cell.

Target: ~3000 ppm (check your specific cell manual — some run 2700 to 3400, others 3200 to 3500). Use only 99.4%+ pure sodium chloride sold as pool salt or solar salt with no rust inhibitors.

Low salt damages the cell. High salt corrodes equipment. Add salt to the shallow end and brush back and forth to dissolve before turning the cell on.

Borates — optional buffer

Borates at 30 to 50 ppm are optional. They improve pH stability and reduce scale formation in salt water systems. Some users report softer water 'feel' but it is subjective.

Raise with granular boric acid. Lower by partial water replacement.

Phosphates — usually irrelevant

Pool stores commonly push phosphate removers as a cure for algae. The truth: if your FC is maintained at the right level for your CYA, phosphate level does not matter. Algae cannot grow when chlorine is doing its job, regardless of phosphate.

Skip phosphate removers unless your FC is balanced and you still have a recurring problem you cannot solve any other way.

The maintenance order that actually works

Test and adjust your pool in this order of importance:

• 1. Test Free Chlorine daily, maintain it based on your CYA
• 2. Track CYA — keep it in range for your sanitizer type
• 3. Keep pH between 7.2 and 7.8
• 4. Monitor Calcium Hardness to prevent corrosion or scaling
• 5. Maintain Total Alkalinity to stabilize pH
• 6. Use salt and borates only if appropriate for your pool
• 7. Skip phosphate removers unless every other parameter is in range and you still have a problem