Pool Water Chemistry: A Practical Guide to Balanced Water

The Five Numbers That Matter

Pool water balance comes down to five measurements. Get these right and your water is clear, safe, and comfortable. Let any one of them drift and problems cascade: algae growth, cloudy water, skin irritation, equipment corrosion, or scale buildup.

Free Chlorine (FC): 2-4 ppm. This is your sanitizer. It kills bacteria, algae, and pathogens. Below 2 ppm and your pool isn't sanitized. Above 5 ppm and it irritates skin and eyes. Test at least weekly, daily during heavy use or hot weather.

pH: 7.2-7.6. This measures acidity. Below 7.2, water is corrosive (etches plaster, corrodes metal, irritates eyes). Above 7.8, chlorine becomes dramatically less effective (only 25% active at pH 8.0 vs 66% active at 7.2), and calcium starts precipitating out of solution as scale. Target 7.4.

Total Alkalinity (TA): 80-120 ppm. This is the pH buffer. If TA is in range, pH stays stable. If TA is low, pH bounces around wildly with every chemical addition or rainstorm. Always fix alkalinity before trying to fix pH.

Cyanuric Acid (CYA): 30-50 ppm for outdoor pools. This is chlorine sunscreen. Without it, UV light destroys half your chlorine within 45 minutes. With CYA at 30-50 ppm, chlorine lasts all day. But too much CYA (above 70-80 ppm) makes chlorine sluggish and unable to kill algae effectively.

Calcium Hardness (CH): 200-400 ppm. This prevents water from becoming corrosive to plaster and equipment. Low calcium water dissolves calcium from your pool surface (etching). High calcium water deposits scale on surfaces and equipment. Less critical in vinyl and fiberglass pools than in plaster.

The Order of Operations

When multiple readings are off, fix them in this order: alkalinity first, then pH, then chlorine, then CYA, then calcium. The reason is dependency: alkalinity controls pH stability, pH controls chlorine effectiveness, and CYA controls chlorine longevity. Fixing them out of order means redoing earlier adjustments.

Always add one chemical at a time. Wait at least 30 minutes (ideally a full pump cycle, typically 6-8 hours) between additions. This lets the chemical circulate and the reading stabilize. Retest before adding the next chemical.

Common Chemicals and What They Do

Liquid chlorine (sodium hypochlorite, 10-12.5%) is the cleanest way to chlorinate. It raises pH slightly and adds no CYA. Buy pool-grade from a pool supply store or use regular unscented household bleach (6-8.25%) at higher volumes.

Trichlor tablets (90% available chlorine) are convenient for daily chlorination via a floating dispenser or inline chlorinator. The catch: every tablet adds CYA to your water. Over months, CYA accumulates above 50+ ppm, reducing chlorine effectiveness. Many experienced pool owners use tablets only for vacations and liquid chlorine for regular dosing to keep CYA in check.

Muriatic acid (31.45% hydrochloric acid) lowers pH and alkalinity. It's the most frequently used adjustment chemical for most pool owners. Handle with care: wear gloves and eye protection, add slowly to deep end with pump running, never pour near pool equipment.

Baking soda (sodium bicarbonate) raises alkalinity with minimal pH impact. Available at any grocery store. Buy the big bag. About 1.5 pounds per 10,000 gallons raises TA by 10 ppm.

Soda ash (sodium carbonate) raises pH. Use when pH is low but alkalinity is fine. About 6 ounces per 10,000 gallons raises pH by 0.2.

Testing: Your Most Important Habit

A quality liquid reagent test kit (Taylor K-2006 or TF-100) is the single best investment in pool maintenance. Test strips are convenient but inaccurate enough to cause real problems. A $10 strip that reads pH 7.4 when the actual pH is 7.8 means your chlorine is 40% less effective than you think it is. Liquid kits cost $50-$80 and last an entire season with weekly testing.

Test weekly during swim season. Test before and after every chemical addition. Test after heavy rain (dilutes chemicals and drops pH). Test after a pool party (bather load consumes chlorine rapidly). Record your readings in a log or phone note. Patterns emerge over time that make maintenance predictable.

The Weekly Routine

A balanced pool requires about 15 minutes per week of active maintenance: test the water (5 minutes), calculate and add chemicals if needed (5 minutes), clean the skimmer basket (2 minutes), and check the pump/filter (3 minutes). That's it. Pool maintenance has a reputation for being complicated, but once you understand the five numbers and how to adjust them, it's genuinely simple.

The pool chemical calculator on this site is designed for exactly this weekly routine. Test your water, enter your readings, and get the exact amounts to add. Bookmark it and use it every week.