The 7 Parameters Every Pool Pro Must Know
Pool water chemistry comes down to seven measurements. Master these and you can diagnose any pool problem.
| Parameter | Ideal Range | What It Does |
|---|---|---|
| Free Chlorine (FC) | 3–5 ppm | Kills bacteria and algae |
| pH | 7.4–7.6 | Controls chlorine effectiveness and comfort |
| Total Alkalinity (TA) | 80–120 ppm | Buffers pH from swinging |
| Calcium Hardness (CH) | 200–400 ppm | Protects surfaces from corrosion |
| Cyanuric Acid (CYA) | 30–50 ppm | Protects chlorine from sunlight |
| Temperature | Varies | Affects chemical reactions and LSI |
| Total Dissolved Solids | < 2,000 ppm | Overall mineral load |
How They Work Together: The Water Balance Triangle
Pool chemistry isn't about perfecting individual numbers — it's about balance between them. The Langelier Saturation Index (LSI) captures this relationship:
- Negative LSI = Corrosive water (dissolves plaster, corrodes metal)
- Zero LSI = Balanced water (ideal)
- Positive LSI = Scaling water (deposits calcium on surfaces)
Every chemical adjustment affects multiple parameters. Raising pH makes water more scale-prone. Lowering alkalinity makes pH less stable. Temperature changes shift the entire balance.
Use our free LSI calculator to see the complete picture.
Free Chlorine: Your Primary Sanitizer
Free chlorine is the workhorse of pool sanitization. It exists in two forms:
- Hypochlorous acid (HOCl) — The powerful killer (more active at lower pH)
- Hypochlorite ion (OCl-) — Weaker form (dominates at higher pH)
At pH 7.4, about 55% of your chlorine is in the powerful HOCl form. At pH 8.0, only 21% is. This is why pH control matters so much.
The FC/CYA Relationship
CYA (stabilizer) protects chlorine from UV degradation but also reduces its killing power. The rule:
Minimum FC = CYA x 0.075 (7.5%)
| CYA Level | Minimum FC | Target FC |
|---|---|---|
| 30 ppm | 2.3 ppm | 3–4 ppm |
| 50 ppm | 3.8 ppm | 4–5 ppm |
| 70 ppm | 5.3 ppm | 6–7 ppm |
| 100 ppm | 7.5 ppm | 8–9 ppm |
If CYA gets above 80 ppm, you're fighting an uphill battle. The only way to lower CYA is to drain and refill.
pH: The Master Variable
pH affects everything:
- Chlorine effectiveness (huge difference between 7.2 and 8.0)
- Swimmer comfort (eyes and skin)
- Surface protection (plaster, metal)
- LSI calculation
Low pH fixes: Soda ash (sodium carbonate) or borax
High pH fixes: Muriatic acid or sodium bisulfate
Read our detailed guides: How to raise pH | How to lower alkalinity
Total Alkalinity: The pH Buffer
TA determines how resistant your pH is to change. Think of it as pH's shock absorber:
- Low TA (below 60 ppm) = pH bounces wildly with every chemical addition
- High TA (above 140 ppm) = pH locks high and resists acid corrections
The sweet spot is 80–100 ppm for most pools.
Calcium Hardness: Surface Protection
Calcium hardness protects pool surfaces:
- Low CH (below 150 ppm) = Water is "hungry" and dissolves calcium from plaster
- High CH (above 400 ppm) = Water deposits calcium as scale
Plaster pools need at least 200 ppm. Vinyl and fiberglass pools are less sensitive but should stay under 400 ppm.
The Professional Testing Routine
At Every Service Stop
- Free chlorine
- pH
- Temperature (for LSI)
Weekly
- Total alkalinity
- Calcium hardness (if your area has variable water)
Monthly
- CYA (stabilizer)
- Salt level (salt pools)
- TDS (if water seems "old")
Common Chemistry Combos and What They Mean
| Symptoms | Likely Cause | Fix |
|---|---|---|
| Cloudy + high pH | High alkalinity | Lower TA with acid |
| Green tint + low FC | Algae beginning | Shock to breakpoint |
| Burning eyes + "chlorine smell" | High combined chlorine | Shock to destroy chloramines |
| Rough plaster + scale on tile | Positive LSI | Lower pH or calcium |
| Etched plaster + corroded fittings | Negative LSI | Raise pH, calcium, or alkalinity |
| Chlorine drops fast | Low or no CYA | Add stabilizer |
Track Everything with PoolOps
PoolOps records all 7 parameters at every service stop, calculates LSI automatically, and tracks trends over time — so you can spot problems before your customers do.