Why Is My Pool Green but the Chlorine Is High?

You’ve tested the water. The chlorine reading is high — maybe 5 ppm, maybe even higher. And yet the pool is green. It doesn’t make sense. Chlorine kills algae, so why isn’t it working?

This is one of the most frustrating situations a pool owner faces, and it has a real explanation. In fact, there are several possible causes, and understanding them can save you from dumping more chemicals into a pool that doesn’t need more chlorine — it needs a different fix entirely.

Cause 1: CYA Lock (Chlorine Lock)

This is the most common reason pools stay green despite high chlorine in the Phoenix area.

Cyanuric acid (CYA) — also called stabilizer or conditioner — protects chlorine from UV degradation. Without it, the Arizona sun would burn through your chlorine in hours. A small amount (30–50 ppm) is essential.

The problem: CYA accumulates over time. Every chlorine tablet (trichlor) you add contains CYA. Every time water evaporates and is replaced, the CYA that was already in the water stays behind. In Phoenix, it’s common to see CYA levels of 150, 200, or even 300+ ppm in pools that haven’t been drained in years.

When CYA is too high, it binds with chlorine and renders it ineffective. Your test kit shows free chlorine is present, but that chlorine is chemically “locked up” by the excess CYA. It can’t oxidize algae or sanitize effectively.

The ratio matters. Industry guidelines suggest maintaining a chlorine-to-CYA ratio of about 7.5% — meaning at a CYA of 50 ppm, you need at least 3–4 ppm of free chlorine. At a CYA of 150 ppm, you’d need 11+ ppm of chlorine just to maintain the same sanitizing power. At 200+ ppm CYA, it becomes practically impossible to maintain enough chlorine.

The fix: There is no chemical that removes CYA from pool water. The only solution is dilution — a partial or full drain and refill. Once CYA is back in the 30–50 ppm range, your chlorine will work normally again.

Cause 2: Metals in the Water

Green water isn’t always algae. Copper dissolved in pool water can give it a green or teal tint that looks exactly like an algae bloom but doesn’t respond to chlorine the way algae would.

Sources of copper in pool water:

• Copper-based algaecides — Ironically, some algaecides introduce dissolved copper that can cause green discoloration
• Copper heat exchanger corrosion — If your pool heater has a copper heat exchanger and the water is acidic (low pH), it dissolves copper into the water
• Source water — Some Phoenix-area well water contains trace copper
• Copper ionizers — Pool systems that use copper ions for sanitation can over-dose if not calibrated properly

How to tell the difference: Algae makes water cloudy and clings to surfaces. Metal discoloration makes water clear but tinted green, and you may see green staining on plaster or around fittings.

The fix: Test for metals specifically (a standard chlorine/pH test won’t show this). If copper is elevated, use a metal sequestrant to bind the copper and prevent staining, then address the source. Do not shock a pool with high metals — oxidizing dissolved metals drives them out of solution and onto your plaster as permanent stains.

Cause 3: Phosphates

Phosphates are a nutrient source for algae. They enter pool water through:

• Landscaping runoff (fertilizers)
• Decomposing organic matter (leaves, pollen)
• Municipal water supply
• Some pool chemicals

High phosphate levels (1,000+ ppb) can fuel algae growth even when chlorine levels seem adequate. The algae essentially has an unlimited food supply, and the chlorine is working overtime trying to kill growth that keeps regrowing.

The fix: Test phosphate levels (a separate test from standard chemistry). If elevated, use a phosphate remover — typically a lanthanum-based product that binds phosphates and drops them out of solution. Your filter will then trap the precipitate. Expect the water to cloud temporarily during treatment.

Cause 4: Filtration Problems

Chlorine can only kill what it contacts. If your filtration system isn’t circulating and filtering effectively, algae can persist in dead zones and areas with poor water flow.

Common filtration issues:

• Dirty or worn filter media — A cartridge filter past its service life, DE grids with holes, or channeled sand all reduce filtration efficiency dramatically
• Insufficient run time — The pump needs to turn over the entire pool volume at least once per day. In summer, twice is better.
• Poor circulation — Return jets pointed the wrong direction, clogged impeller, or a pump running at too low a speed
• Undersized equipment — A filter or pump that’s too small for the pool volume

The fix: Clean or replace the filter media. Verify the pump is running enough hours. Check that return jets create good surface movement and push water toward the skimmer and main drain.

The Diagnostic Sequence

When your pool is green but chlorine tests high, work through this checklist:

1. Test CYA — If over 80 ppm, this is likely your primary problem. Drain and refill.
2. Test for metals — If copper is present, treat with sequestrant before shocking.
3. Test phosphates — If over 500 ppb, treat with phosphate remover.
4. Inspect the filter — Clean or replace media, verify pressure readings, confirm adequate run time.
5. Then shock — Once the underlying issue is resolved, a proper shock treatment will clear the remaining algae.

Adding more chlorine without addressing the root cause is like putting gas in a car with a flat tire. The fuel isn’t the problem.

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Dealing with a stubborn green pool in Phoenix? Contact Splash Mob Pools — we diagnose the real problem, not just the symptoms.