Pool Chemistry Guide

Step 1: The Six Levers of Pool Chemistry

Every pool-chemistry decision comes down to six numbers. Not five. Saltwater pools need a sixth lever most guides skip. Here's the full list and why each one matters.

1. Free Chlorine (FC). This is your sanitizer. It's the only number that has to be right every day. Low FC and the pool becomes unsanitary within hours. The target is not a fixed 2 to 4 ppm. It depends on CYA (see Step 2).

2. pH. Target 7.4 to 7.6. pH affects how much of your FC is active sanitizer (HOCl) versus inactive (OCl−). At pH 7.0, about 75% of FC is HOCl. At pH 8.0, only 3%. A high-pH pool with "good" FC readings is still a poorly-sanitized pool.

3. Total Alkalinity (TA). Target 80 to 120 ppm. TA buffers pH against change. If TA is low, pH swings unpredictably with every dose and every rainstorm. Fix TA first. pH stability depends on it.

4. Calcium Hardness (CH). Target 200 to 275 ppm for plaster and concrete. 175 to 225 for vinyl and fiberglass. Too low corrodes. Too high scales. Feeds into LSI water balance.

5. Cyanuric Acid (CYA). Chlorine stabilizer. Protects FC from UV destruction. Target 30 to 50 ppm for outdoor chlorine pools; 60 to 80 for SWG pools; 0 for indoor pools. CYA is the number that makes FC targets variable instead of fixed.

6. Salt (SWG pools only). Salt concentration feeds the electrolysis cell. Target varies by generator: Pentair 3,600 ppm ideal, Hayward 3,200, Jandy 3,000. Always confirm against your specific model's installation manual. Too low damages the cell. Too high corrodes metal fittings.

Those are the six. Not five. Any guide that teaches five is missing salt, which means it's written for someone else's pool. Three-quarters of new pools sold in the US since the mid-2010s are saltwater. The sixth lever matters.

Step 2: The FC:CYA Ratio (The Insight Most Guides Miss)

This is the single most important concept in pool chemistry. It's also the one that most pool-store employees get wrong.

The rule: minimum FC must be at least CYA divided by 7.5. Target FC is the minimum up to roughly 1.5 times that.

This ratio exists because CYA binds reversibly to chlorine. It protects FC from UV destruction, but it also holds some FC in reserve form where it can't sanitize at that instant. The higher your CYA, the more total FC you need to keep active sanitizer available.

Worked examples:

This table is the reason "I follow the pool-store directions and my pool still goes green" is such a common homeowner complaint. The pool store is selling you the 2 to 4 ppm FC target that applies to a pool at CYA 0, then selling you stabilized chlorine tabs that push your CYA up to 70 or 80, and the math stops working.

SLAM (Shock, Level, and Maintain) is the emergency protocol that applies when CC rises above 0.5 ppm or the pool turns green. SLAM FC targets are also CYA-keyed, and go much higher:

Drop pH to 7.2 before starting SLAM. At CYA 75 or higher, drain 30 to 50 percent of the pool before starting. Hold the target until all three exit tests pass: CC below 0.5, overnight FC loss below 1 ppm, water visibly clear.

Step 3: The Strategic Chlorine Choice

The chlorine type you use isn't a preference. It's a strategic decision that determines your CYA drift, CH drift, and monthly cost.

Liquid chlorine (sodium hypochlorite, 10 to 12.5%). No CYA side effect. No CH buildup. Raises pH slightly. The cleanest daily sanitizer once CYA is already in range. Dose: 13 fl oz per 10,000 gallons per 1 ppm FC (10% strength).

Cal-hypo (calcium hypochlorite, 65 to 73%). Adds 0.8 ppm CH per 1 ppm FC as a side effect. Good for pools with low CH that need calcium anyway. Bad for pools in hard-water regions where CH is already high. Dose: 2.0 oz by weight per 10,000 gallons per 1 ppm FC (65% strength).

Dichlor (sodium dichloroisocyanurate, 56%). Stabilized. Adds 0.9 ppm CYA per 1 ppm FC. Useful for bringing CYA up to target in a new pool. Not for season-long use because CYA will accumulate past target. Dose: 2.4 oz by weight per 10,000 gallons per 1 ppm FC.

Trichlor (trichloroisocyanuric acid, 90%). Stabilized tabs. Adds 0.6 ppm CYA per 1 ppm FC generated. Convenient via floater or inline feeder. The catch: a full season of trichlor pushes CYA up 30 to 50 ppm. Use for vacation automation only, or for the first month of a new pool. Switch to liquid once CYA hits 50. Dose: 1.5 oz by weight per 10,000 gallons per 1 ppm FC.

Step 4: The Adjustment Order

When multiple levers are off, fix them in this order:

1. Total Alkalinity first. TA is the pH buffer. Unstable TA means pH keeps drifting back no matter how much acid or soda ash you add.
2. pH second. pH affects chlorine effectiveness and LSI balance. Adjust it after TA is stable.
3. Calcium Hardness third. Slower drift timeline. Adjust once pH is in range.
4. Cyanuric Acid fourth. Slowest lever. Takes 48 to 72 hours to dissolve. Don't re-test FC until CYA has settled.
5. Chlorine last. Because CYA sets the FC target (see Step 2). Dose FC only after you know what your CYA is.

One chemical at a time. Wait between additions. Retest before the next adjustment.

The Arm & Hammer secret

Pool-store alkalinity increaser is sodium bicarbonate, 100% pure. So is Arm & Hammer baking soda. Costco and Sam's Club sell 13.5 lb bags for about $8 to $10. The equivalent pool-store bag runs $22 or more. Same chemical. Same ppm rise. Roughly 30% of the price. 1.5 lb per 10,000 gallons raises TA by 10 ppm.

Don't overthink this. It's the same chemical.

Lowering TA without losing pH

Lowering TA requires the acid-and-aerate method. Two steps, in this order:

1. Dose muriatic acid. This drops pH and TA together. 25.6 fl oz per 10,000 gallons per 10 ppm TA drop.
2. Aerate the pool. Point returns upward, run fountains, add aerators. Aeration outgasses CO2, which raises pH without re-raising TA.

Never aerate first. Aeration with no prior acid dose cannot lower TA; it only raises pH.

Step 5: Testing Discipline

A FAS-DPD drop kit (Taylor K-2006 or equivalent) is the single best pool investment. Test strips are convenient but inaccurate enough to cause real problems. A strip that reads pH 7.4 when actual pH is 7.8 means your chlorine is 40% less active than you think. A kit costs $50 to $80 and lasts a full season at weekly testing.

Test weekly during swim season at minimum. Test before and after every chemical addition. Test after heavy rain (dilutes chemistry). Test after a pool party (bather load consumes chlorine). Log the readings; patterns emerge that make maintenance predictable.

Common mistakes (from reader pain patterns)

• Running trichlor tabs all season. CYA climbs past 80 by August, FC binds to stabilizer, pool goes green. Switch to liquid once CYA hits 50.
• Following a 2 to 4 ppm FC target at any CYA. Guarantees an unsanitized pool at CYA 50+.
• Shocking without checking CYA first. If CYA is 100+, shocking to "just dump in a couple bags" produces no actual disinfection. Drain first.
• Aerating first to lower TA. Chemically impossible. Aeration raises pH. It can only restore pH after acid lowered it.
• Buying pool-store alkalinity increaser. Same chemical as grocery-store baking soda at roughly 3x the price.
• Dumping cal-hypo directly on a vinyl liner. Bleaches the pattern where it lands. Pre-dissolve in a bucket and pour across the return.
• Ignoring the SWG brand-specific salt target. Pentair runs 3,600. Hayward runs 3,200. Jandy runs 3,000. The manufacturer's manual beats the internet average.