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Pool Chemistry 101

Understand the WHY behind every chemical adjustment. The foundation of pool care.

When to use: First-time pool owner who needs to understand FC/CYA, pH/TA, and water balance basics.
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Pool Chemistry 101: Understanding the Fundamentals

Learn the WHY behind every adjustment. This knowledge will save you thousands of dollars and countless hours.

The Philosophy: Science Over Sales

Pool stores make money selling you products to fix problems. We teach you to PREVENT problems with science. Once you understand these fundamentals, you'll spend 80% less on chemicals and have clearer water than your neighbors who rely on 'magic potions'. The approach is simple: test, calculate, adjust. No gimmicks.

1

The Big Picture: What You're Actually Managing

Pool water chemistry isn't rocket science - it's just 6 numbers that work together.

FC (Free Chlorine): Your sanitizer - kills bacteria, algae, and viruses
pH: Measures acidity/alkalinity - affects how well chlorine works and swimmer comfort
CYA (Cyanuric Acid): Sunscreen for chlorine - protects it from UV degradation
TA (Total Alkalinity): pH buffer - prevents rapid pH swings
CH (Calcium Hardness): Calcium content - prevents corrosion or scaling
CSI (Calcium Saturation Index): Calculated value - tells you if water will eat or scale surfaces
Key Insights
  • • FC and pH are your daily drivers - test these 2-3× per week minimum
  • • CYA is the 'set it and forget it' parameter - rarely changes once established
  • • TA, CH, and CSI are supporting actors - check weekly or when troubleshooting
  • • Every parameter affects others - this is a system, not isolated values
2

The FC/CYA Relationship: The Most Important Concept

This is the foundation of effective pool chemistry. Get this right and everything else is easier.

CYA protects chlorine from UV - without it, sunlight destroys FC in 2 hours
BUT: CYA also 'shields' bacteria/algae from chlorine - you need MORE FC as CYA increases
The Rule: FC should be at least 7.5% of your CYA level (minimum)
Example: CYA 40 ppm → FC minimum is 3 ppm (40 × 0.075 = 3)
Example: CYA 50 ppm → FC minimum is 4 ppm (50 × 0.075 = 3.75, round up to 4)
Target Range: Keep FC between 7.5% (minimum) and 12% (high target) of CYA
Poolometer calculates this automatically - you just need to understand WHY
Key Insights
  • • Pool stores often say 'CYA 80-100 is fine!' - it's not. You'll need 6-8 ppm FC minimum, which is expensive
  • • Sweet spot for outdoor pools: CYA 30-50 ppm, FC 3-6 ppm for minimum targets
  • • Indoor pools: CYA 20-30 ppm (less UV exposure)
  • • Salt pools: SWG produces UNstabilized chlorine (same as liquid bleach) - CYA won't increase unless you add stabilized chlorine tablets
💡 Common Real-World Scenario
Problem: My pool keeps getting cloudy or algae starts, but I'm adding chlorine!
Diagnosis: Your CYA is probably too high (70+), so your 3 ppm FC isn't enough. You need 5.25+ ppm minimum.
Solution: Either raise FC to match CYA (expensive) or partially drain and refill to lower CYA (one-time fix).
3

pH and Total Alkalinity: The Buffering System

pH affects chlorine efficiency, equipment lifespan, and swimmer comfort. TA keeps pH stable.

pH measures acidity (low pH) vs alkalinity (high pH) on 0-14 scale
Pool target: pH 7.2-7.8 (7.4-7.6 is ideal for comfort and chlorine efficiency)
pH affects chlorine: At pH 7.0, chlorine is 75% effective. At pH 8.0, only 25% effective!
TA (Total Alkalinity) acts as pH buffer - resists rapid changes
TA target: 60-90 ppm for plaster pools, 80-120 ppm for vinyl/fiberglass
High TA → pH is stubborn and drifts upward constantly
Low TA → pH swings wildly with every small chemical addition
Key Insights
  • • Always adjust TA BEFORE fighting pH - TA is the foundation
  • • If pH keeps rising: Lower TA using muriatic acid (add slowly over days)
  • • If pH drops rapidly: Raise TA using baking soda (sodium bicarbonate)
  • • Aeration (fountain, waterfall, jets) raises pH naturally - use this instead of chemicals when possible
💡 Common Real-World Scenario
Problem: I keep adding pH Down but pH goes back up to 8.0 within 2 days!
Diagnosis: Your TA is too high (120+). The buffer is so strong it's pulling pH back up.
Solution: Lower TA to 70-80 ppm using muriatic acid. Once TA is right, pH will stay stable.
4

Calcium Hardness: Preventing Corrosion and Scale

CH is the calcium content in your water. Too low = corrosion, too high = scale.

CH measures dissolved calcium in parts per million (ppm)
Target: 250-400 ppm for plaster pools, 200-300 ppm for vinyl/fiberglass
Low CH: Water seeks calcium and dissolves it from plaster, grout, equipment (expensive damage)
High CH: Excess calcium precipitates out as scale on surfaces, tile, and in pipes
CH rarely drops on its own - only from draining/refilling or rainwater dilution
CH rises from: Hard fill water, calcium-based chlorine products, CH increaser
Key Insights
  • • Test CH monthly - it changes very slowly
  • • If fill water is 400+ ppm, you'll fight scale forever. Consider reverse osmosis fill or softer source.
  • • Lowering CH requires partial drain and refill - no chemical can remove it
  • • Raising CH is easy - add Calcium Chloride (fast) or let calcium-based chlorine raise it over time
💡 Common Real-World Scenario
Problem: White crusty deposits on tile line and inside pump basket. Water is cloudy after shocking.
Diagnosis: CH is too high (500+ ppm) and water is supersaturated with calcium. Scale is forming.
Solution: Partially drain and refill with low-CH water. Target 300 ppm after refill.
5

CSI (Calcium Saturation Index): Water Balance

CSI combines pH, TA, CH, and temperature to predict if water will corrode or scale surfaces.

CSI is calculated from: pH, TA, CH, water temperature, and TDS (Total Dissolved Solids)
Poolometer calculates this automatically - you don't need to do math
CSI = 0: Perfectly balanced water (rare, don't obsess over this)
CSI -0.3 to +0.3: Safe zone - water is balanced
CSI < -0.3: Corrosive water - will dissolve calcium from surfaces and equipment
CSI > +0.3: Scaling water - will deposit calcium as crusty white scale
Key Insights
  • • CSI drifts positive in summer (warm water + evaporation) - lower pH slightly to compensate
  • • CSI drifts negative in winter (cool water + rain dilution) - raise pH/CH if needed
  • • You control CSI by adjusting pH (fastest), TA (slower), or CH (slowest)
  • • Warmer water makes CSI more positive - expect scale pressure in July-August
💡 Common Real-World Scenario
Problem: Poolometer says CSI is +0.5 and water is scaling. What do I do?
Diagnosis: Water is supersaturated with calcium and will deposit scale.
Solution: Lower pH to 7.2 temporarily (fast fix) OR lower TA to 60-70 ppm (slow fix). Monitor CSI weekly in summer.
6

The Chlorine Demand Test: Is Your Pool 'Clean'?

This test tells you if your pool is truly sanitized or just looks clear.

Chlorine demand = how fast your pool consumes chlorine overnight
High demand = organic contamination (algae, bacteria, oils, pollen)
OCLT (Overnight Chlorine Loss Test): Test FC at dusk, then again at dawn (pump running)
Good pool: Loses <1 ppm FC overnight (stable, clean)
Problem pool: Loses 2-4+ ppm FC overnight (contamination present, needs SLAM)
This test finds invisible problems BEFORE water turns green
Key Insights
  • • Run OCLT after heavy rain, parties, or if water looks 'dull'
  • • If OCLT fails, proceed to SLAM process (Shock Level And Maintain)
  • • A passing OCLT means you can swim confidently - the water is truly sanitized
  • • Pool stores never mention this test - it would reveal most 'clear' pools aren't actually clean
7

Understanding Chemical Adjustments: Add, Wait, Test

The biggest beginner mistake: adding too much, too fast. Pool chemistry needs time.

Rule #1: Add 75% of calculated dose, wait 30+ min, retest, then add remainder if needed
Rule #2: Only adjust ONE parameter at a time (exception: TA+pH, which are linked)
Rule #3: Test BEFORE adding chemicals - don't guess based on 'how it looks'
Rule #4: Circulate 30 min minimum after adding anything - water must mix
Rule #5: Log everything - patterns emerge over weeks/months
Key Insights
  • • Liquid chlorine (bleach) is easiest to dose precisely - splash it in deep end with pump running
  • • Muriatic acid is STRONG - always add to water, never water to acid. Pour slowly in deep end.
  • • Baking soda (TA up) and Calcium Chloride (CH up) dissolve slowly - broadcast over surface with pump on
  • • Borax (pH up without affecting TA) is advanced - stick to basics for first season
8

Putting It All Together: The Weekly Routine

Now that you understand the WHY, here's the practical WHAT to do each week.

Test FC + pH (2-3× per week, more in summer or after rain/parties)
Adjust FC if below minimum target (add liquid chlorine or run SWG longer)
Adjust pH if outside 7.2-7.8 range (muriatic acid to lower, aeration to raise)
Test TA + CH (weekly or when pH is misbehaving)
Adjust TA if pH won't stay stable (lower with acid, raise with baking soda)
Check CSI in Poolometer (weekly in summer, monthly in winter)
Run OCLT if water looks dull or FC drops faster than expected
Key Insights
  • • Morning testing (before sun hits pool) gives most accurate FC readings
  • • Keep a testing kit by the pool - easier to test = you'll actually do it
  • • Use Poolometer to log tests - automatic analysis catches problems early
  • • Join online pool chemistry communities for support and advice from experienced owners

Congratulations! You now understand more about pool chemistry than 90% of pool owners. The next step is practice - test your pool, log the results in Poolometer, and watch how these concepts play out in real life. Come back to this guide whenever you need a refresher. You've got this! 🎓

Checklist

  1. 1Understand the 6 core parameters and what they do.
  2. 2Master FC/CYA relationship - the foundation of effective pool chemistry.
  3. 3Learn pH and TA buffering system.
  4. 4Understand calcium hardness and scale prevention.
  5. 5Calculate CSI for water balance.
  6. 6Run chlorine demand test (OCLT) to verify sanitation.
  7. 7Learn safe chemical adjustment practices.
  8. 8Build your weekly testing routine.

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