Pool Water Chemistry: The Complete Picture

Pool water chemistry is more than just “add some chlorine and hope for the best.” Good chemistry protects swimmers, protects the surface, keeps equipment alive, and keeps the water looking clear instead of cloudy or green. This page gives a broad overview of the system so the detailed sections on chlorine, pH, CYA, and surface care all make sense in context.

What Good Water Chemistry Actually Does

• Sanitization: Kills germs quickly enough to prevent recreational water illnesses.
• Clarity: Keeps water visually clear so you can see the bottom and spot hazards.
• Surface protection: Prevents etching, scaling, and discoloration of plaster and tile.
• Equipment protection: Avoids corrosion of heaters and metal parts, and scale in pipes and salt cells.
• Comfort: Reduces eye and skin irritation and harsh odors from combined chlorine.

The Three Pillars: Chemistry, Circulation, Filtration

Water quality is not just chemicals. You need all three:

• Chemistry: Proper levels of free chlorine, pH, alkalinity, calcium hardness, and CYA to keep the water balanced and disinfecting.
• Circulation: Moving water past the skimmer, main drain, and returns so dead spots are minimized and chemicals are mixed evenly.
• Filtration: Physically removing fine particles so the water stays clear and the sanitizer doesn’t waste its strength burning off debris.

Poor circulation and filtration can make “good chemistry on paper” perform badly in the real pool. Aim for complete turnover of the pool volume within the time recommended by your local code or manufacturer guidance, with the pump and returns adjusted to minimize dead zones.

Key Testing Parameters

A good test kit is the starting point. Strips can be useful for a quick check, but for any serious diagnosis or adjustment you should rely on a drop-based test kit that can measure free chlorine, combined chlorine, pH, alkalinity, calcium hardness, and CYA.

Free Chlorine (FC)

Free chlorine is the active sanitizer that kills bacteria, viruses, and algae. In most pools, a practical target is about 1–3 ppm, adjusted for bather load, CYA level, and local regulations. The detailed Chlorine Dosing page covers how to calculate dosing and neutralizer.

Combined Chlorine (CC)

Combined chlorine is chlorine that has already reacted with contaminants to form chloramines. High CC causes “chlorine smell” and irritation. CC above about 0.5 ppm usually indicates a need for breakpoint chlorination. Managing CC is covered under Chlorine Dosing & Sanitizing Pools.

Total Chlorine (TC)

Total chlorine is simply FC + CC. Many basic tests give TC and let you calculate CC by subtraction. TC by itself doesn’t tell you if the water is safe; you need to know how much of that total is still free, active sanitizer.

pH

pH controls both comfort and chemistry. It changes how strong chlorine is, how likely calcium is to deposit or dissolve, and how aggressive the water is toward plaster and metal. Most pools operate best between 7.2 and 7.6. The pH Balance page explains how pH and alkalinity interact, and how to use acid, soda ash, and bicarbonate correctly.

Total Alkalinity (TA)

Total alkalinity is the water’s buffering capacity—how resistant it is to sudden pH swings. For many plaster systems, 80–120 ppm is a common range, but the ideal value depends on pH, calcium, and overall balance. Too low, and pH bounces around wildly. Too high, and pH drifts upward constantly.

Calcium Hardness (CH)

Calcium hardness measures how much calcium is dissolved in the water. For plaster pools, CH that is too low encourages the water to pull calcium out of the surface (etching), while too high encourages scale on the tile line, heaters, and salt cells. Typical ranges are roughly 200–400 ppm depending on local fill water and temperature. The Surface Care page ties CH directly to plaster health.

Cyanuric Acid (CYA)

CYA is the stabilizer or conditioner that protects chlorine from sunlight. It is indispensable in outdoor pools but can easily be overdone. Low CYA lets the sun burn off chlorine too fast. High CYA slows disinfection and can make the water difficult to remedy after contamination. The Conditioner / CYA page explains the chemistry, dosing formulas, and CYA-to-chlorine relationships in detail.

Temperature and TDS

Temperature affects both comfort and LSI. Warm water tends to be more scale-forming for the same chemistry. Total dissolved solids (TDS) accumulate over time as chemicals and contaminants build up. High TDS doesn’t always cause problems by itself, but it can indicate that the water is “old” and ready for partial drain and refill, especially in heavy-use or heavily-dosed pools.

Langelier Saturation Index (LSI)

The Langelier Saturation Index combines pH, alkalinity, calcium hardness, temperature, and TDS into a single number that predicts whether the water will dissolve calcium (negative LSI) or deposit it as scale (positive LSI). For plaster, a common target is around 0.0 to +0.3:

• Too negative: Water is aggressive; it tends to etch plaster and grout, and strip metals.
• Too positive: Water is scale-forming; calcium can deposit in heaters, pipes, tile lines, and cells.

LSI is a tool, not a religion. Use it to understand why a pool with “perfect” pH and calcium on paper might still be etching (low temperature, low alkalinity) or scaling (very high calcium and warm water).

Water Clarity & Visual Safety

Sanitizer and balance are not enough if the water is cloudy. Clarity matters for:

• Safety: Being able to see the bottom at the deepest point.
• Diagnostics: Cloudiness can indicate poor filtration, algae bloom, or high fine-particle load.
• Public perception: Clear water is the first sign, to most people, that the pool is “clean.”

If chemistry looks good but the water is dull or cloudy, look at run times, filter condition, and circulation patterns alongside your test results.

Testing Frequency and Habits

A reasonable starting point for most outdoor pools is:

• Daily or every visit: Free chlorine, pH.
• Weekly: Alkalinity, calcium hardness (if not yet stable), and CYA in season.
• After major events: Heavy bather load, storms, large chemical additions, or algae treatment.

Always test before dosing. For large corrections, use split doses: add part of the calculated amount, circulate, retest, and adjust. This avoids overshooting and teaches you how the specific pool responds.

Dosing: From Test Result to Chemical Amount

All dosing follows the same basic pattern:

1. Measure the current level with a reliable test.
2. Decide on a realistic target range.
3. Compute the ppm change needed (Δppm).
4. Use a known “dose per 10,000 gallons” factor for the product you’re using.
5. Scale the dose to your actual pool volume.

The Chlorine Dosing and pH Balance pages walk through real-world examples with step-by-step math for chlorine, neutralizer, acid, bicarbonate, and soda ash.

Long-Term Care

Good water chemistry is not a one-time event; it is a pattern over months and years. Long-term care includes:

• Monitoring CYA creep from stabilized chlorines and planning water replacement before it gets excessive.
• Adjusting calcium hardness as needed to keep LSI in range as seasons and temperatures change.
• Watching for early signs of etching or scaling on steps, tile, and fittings.
• Aligning chemistry with plaster curing needs, especially in the first 30 days for new surfaces.

For surface-specific guidance, see Plaster Surface Care, which applies these chemistry concepts directly to startup and long-term protection of cement-based finishes.