PoolChem Tracker

Pool water green but chlorine is high? Here's why

You tested your water, your chlorine reads fine — maybe even high — but the water is green. This is one of the most frustrating pool problems because it seems like the chemistry should be working. Here's what's actually happening.

The 4 most likely causes

Cause 1: High CYA (the most common culprit)

This is the #1 reason for "green water with good chlorine." Cyanuric acid (CYA) protects chlorine from UV sunlight, but when CYA gets too high, it binds to so much of your free chlorine that there's almost nothing left to actually sanitize.

Your test kit shows FC of 4 ppm — looks fine. But if your CYA is 100 ppm, only a tiny fraction of that FC is actually active. The effective chlorine level is far lower than what the test reads.

How to fix it:

• Test your CYA level. If it's above 70-80 ppm, that's likely your problem
• The only way to lower CYA is to drain and refill part of the pool. There's no chemical that removes it
• Drain 1/3 to 1/2 of the water, refill, and retest CYA
• Once CYA is in the 30-50 ppm range, shock the pool to kill the algae

Cause 2: Metals in the water

Here's the twist: green water doesn't always mean algae. Dissolved copper or iron can tint pool water green, blue-green, or brown — and chlorine won't fix it because it's not a biological problem.

Clues that it's metals, not algae:

• The water is tinted but clear (you can see the bottom) — algae makes water cloudy
• The green appeared right after adding chlorine, shocking, or adjusting pH
• You use well water or have copper plumbing/fittings
• You recently used a copper-based algaecide
• There are green, brown, or blue stains on pool surfaces

How to fix it:

• Test for copper and iron (a pool store can do this, or use a metals test kit)
• Use a metal sequestrant — this binds to dissolved metals and keeps them in solution so they don't stain or discolor
• Do NOT shock until metals are treated. Oxidizing metals makes staining worse
• Consider a hose pre-filter when adding water if your source has high metals

Cause 3: False chlorine reading

Some test kits and strips can give misleading chlorine readings. The most common issue:

• DPD bleach-out: When chlorine is extremely high (above 10-15 ppm), DPD reagent can bleach clear instead of turning dark pink. The test reads zero or low — when chlorine is actually off the charts. This usually happens during or right after shocking
• Reading TC instead of FC: Some cheap test strips only measure total chlorine. If you have high combined chlorine, TC looks fine but FC could be near zero
• Expired reagents: Old or heat-damaged reagents give inaccurate readings

How to fix it:

• Dilute your sample 50/50 with tap water and retest — if the reading jumps up, you had DPD bleach-out
• Make sure you're reading free chlorine, not total
• Replace reagents or strips if they're more than a year old
• Consider upgrading to a FAS-DPD drop test — it doesn't suffer from bleach-out

Cause 4: Algae outpacing your chlorine

Sometimes the chlorine reading is genuinely correct, but algae is growing faster than your chlorine can kill it. This happens when:

• Phosphate levels are very high (algae food source)
• The pool had a long period of neglect and algae has a deep foothold
• Water temperature is very high (above 85°F), which accelerates algae growth and chlorine consumption simultaneously
• You have dead spots with poor circulation where algae can shelter

How to fix it:

• Brush the walls and floor thoroughly — algae clings to surfaces and forms a protective layer that chlorine can't penetrate without disruption
• Shock aggressively — bring FC to at least 30 ppm for a true algae kill (this is called SLAM: Shock Level And Maintain)
• Run the pump 24/7 until the water clears
• Clean or backwash the filter frequently during the process — it's catching a lot of dead algae
• Test and maintain FC every few hours during active treatment

Decision tree: diagnose your green water

1. Test CYA. If it's above 70 ppm → partially drain, refill, then shock
2. Is the water clear but green-tinted? → Likely metals. Test for copper/iron. Use sequestrant
3. Retest chlorine with dilution method. If reading jumps → DPD bleach-out. Your FC is actually very high — just wait
4. All of the above check out? → Aggressive algae. Brush, SLAM shock to 30 ppm, run pump 24/7, filter frequently

Keep reading

• Pool Chlorine Levels Chart — understand what your FC, TC, and CC readings mean
• Free Chlorine vs Total Chlorine — why the gap between them matters
• Why Is My Pool Water Cloudy? — the 5 causes and fixes for cloudy water
• Pool Chemistry for Beginners — the 5 numbers every pool owner should track