Pool Care Blog
Pool Is Green But Chlorine Is High — What's Going On?
Pool Problems · March 2026 · 5 min read
You pull out your test kit. Chlorine reads 5 ppm — higher than normal. But your pool is still green. This is one of the most confusing situations for Houston pool owners, and it reveals a critical misunderstanding about how chlorine actually works.
High chlorine doesn't always mean a clean pool. Sometimes high chlorine means your pool's chemistry is fundamentally broken.
The Chlorine Paradox: High Readings, Green Water
When chlorine kills algae, it gets consumed — converting from "free chlorine" (the active disinfectant) to "combined chlorine" (the spent, inactive form). If your test shows high total chlorine but the water is green, here's what's happening: your chlorine readings include combined chlorine, which does nothing to stop algae.
Free Chlorine: The active stuff that kills algae and bacteria.
Combined Chlorine: The "used up" chlorine that's already done its job (or failed to). Creates that harsh chlorine smell and yellows the water.
Your standard pool store test kit doesn't distinguish between them — it just adds both numbers. A pool with 5 ppm total chlorine might have only 1 ppm free chlorine and 4 ppm combined. That's insufficient to stop algae.
Red Flag: Combined Chlorine Above 0.5 ppm
If combined chlorine is high, your pool needs shock treatment immediately. This is more critical than total chlorine levels.
The Real Culprit: CYA Lock
Most likely explanation in Houston: CYA lock. Cyanuric acid (stabilizer) accumulates to dangerous levels because of year-round tablet use. When CYA exceeds 80 ppm (often 100+ in Houston), chlorine becomes chemically bound to the stabilizer and loses effectiveness — even at high readings.
Your test shows 6 ppm total chlorine. But with 120 ppm CYA, that chlorine is mostly locked up. You need 4+ ppm free chlorine for algae control when CYA is that high — and you only have 1–2 ppm active.
CYA to Chlorine Ratio Chart (Houston Standard)
| CYA Level | Min. Free Chlorine |
|---|---|
| 30–50 ppm | 1–3 ppm |
| 50–80 ppm | 2–4 ppm |
| 80+ ppm (CYA Lock) | 4–6 ppm (often insufficient) |
When CYA is above 80, chlorine is chemically inhibited. No amount of added chlorine will fully compensate. The only real solution is partial drain and refill.
Other Reasons for Green Water Despite High Chlorine
Metals in the Water (Copper, Iron)
Old pipes, well water, or metal fittings leach copper and iron. These metals oxidize in the presence of chlorine and turn the water green or brown. Chlorine readings look normal, but the pool is green because of metal content, not algae. Solution: sequestrant chemical or professional metal removal.
Dead Algae Still Suspended
Chlorine killed the algae 24 hours ago, but the dead algae cells are still floating. Water hasn't cleared yet. The chlorine did its job — you just need to filter out the debris. Run the pump 24–48 hours more and brush aggressively.
Phosphates (Algae Food)
Rain runoff in Houston brings phosphates. Algae feeds on phosphates even when chlorine is present. High chlorine can't prevent algae if phosphates are 1,000+ ppb. Solution: phosphate test and removal chemical if needed.
pH Too High
When pH exceeds 7.8, chlorine's effectiveness plummets. You can have 5 ppm chlorine and still lose the algae fight if pH is 8.0+. Lower pH to 7.2–7.6 and chlorine becomes active again.
How to Properly Test (Not Just Test Strips)
Pool store test strips are notoriously inaccurate. If you're relying on them, you might not know your real chlorine or CYA levels. Here's what professional testing includes:
Free Chlorine — Active disinfectant (reagent test kit)
Combined Chlorine — Spent chlorine (calculated from total minus free)
pH — Water acidity (electronic meter or reagent)
Alkalinity — Buffering capacity
CYA (Stabilizer) — Sunscreen level (critical in Houston)
Phosphates — Algae food source (optional but recommended)
Metals — Copper and iron (if water discolored)
A professional test costs $20–50. Admiral includes full testing with every service visit. Knowing your actual numbers changes the entire treatment plan.
What to Do Right Now
Get a Professional Test
Don't trust the test kit. Bring a water sample to Admiral or another certified pool service. We'll identify the actual problem.
Stop Adding Chlorine
If you have high free chlorine (3+ ppm) and the pool is still green, chlorine isn't the issue. More chlorine won't help.
Brush & Circulate
Run your pump 24/7 and brush walls/floor. If it's dead algae, filtration will clear it within 48 hours.
Check Your CYA Level
If CYA is above 80 ppm, that's your problem. Chlorine is locked up. A partial drain (25–50%) is the only real fix.
Confused about your pool chemistry? We'll test and diagnose the real problem.
Get a Professional Test →Need Professional Pool Help?
Admiral Pool Service has been keeping Greater Houston pools crystal clear since 2019. Licensed, insured, and PHTA certified.
Or call (281) 624-6191.
More from the Admiral pool care blog · RAIL-licensed · serving Greater Houston since 2019.