Trichlor vs. Dichlor vs. Cal-Hypo: Which Pool Chlorine Is Right for You?
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A green pool looks alarming, but green algae is the most common — and most treatable — type. With a brush, a heavy dose of shock, and a running filter, you can go from green to clean in a few days. Here's the exact process.
Few things make a pool owner's heart sink like walking out to green water. The good news: green algae is beatable at home, and you almost never need to drain the pool. Algae is a plant, and like any plant it dies without the right conditions — in this case, a strong enough dose of chlorine and water that keeps moving. Follow the steps below in order and most green pools turn clear within a few days.
What causes green algae in a pool?
Green algae blooms the moment your sanitizer can't keep up. The most common triggers:
- Free chlorine dropped to zero — the number-one cause, whether from a missed week, a heat wave, or a pump that stopped running.
- Stabilizer (cyanuric acid) too high — excess CYA "locks" chlorine so it can't work, even when the number looks okay.
- Poor circulation — dead spots behind ladders and in corners give algae a foothold.
- Warm, sunny weather and heavy rain — heat speeds algae growth, and rain dilutes chlorine while adding organic food.
Because algae reproduces fast, a pool can go from clear to green in 24 to 48 hours once chlorine bottoms out — which is why it so often seems to happen "overnight."
Before you start: test and gather supplies
Test the water first so you know what you're working with — free chlorine, pH, alkalinity, and cyanuric acid. Then bring pH down toward 7.2–7.4. This matters: chlorine is far more effective at killing algae at the lower end of the range, so this small step makes your shock hit much harder.
Have these ready before you begin: pool shock (cal-hypo works well for algae), a quality algaecide for afterward, a stiff pool brush, your test kit, and a clean filter.
Why pH first
Balancing pH to about 7.2 before shocking can dramatically improve results. Skip it and a lot of your chlorine is wasted fighting high pH instead of killing algae.
The 6-step green-to-clean method
- Brush everything. Brush the walls, floor, steps, and every corner. Breaking up the algae exposes it to chlorine and stops it hiding on surfaces.
- Balance pH to ~7.2–7.4. Lower pH makes the coming shock far more effective.
- Shock hard, at dusk. Add a heavy dose (see the severity table below) after the sun goes down so it isn't burned off. This is the step that kills the bloom.
- Run the filter 24/7. Keep the pump running continuously to circulate chlorine and pull dead algae out. Clean or backwash the filter as it loads up.
- Brush and vacuum again. As the green turns cloudy gray or white, the algae is dying. Brush once more, then vacuum the dead algae from the floor. Re-shock if any green remains.
- Add algaecide and resume your routine. Once free chlorine settles back to 1–3 ppm, add a preventive algaecide and return to normal dosing. Adding it now — rather than during the high-chlorine phase — keeps the algaecide from being wasted.
The green-to-clean essentialsPool shock kills the bloom and algaecide keeps it from returning — the two products every algae fight needs.
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How much shock? (by severity)
The greener the water, the more shock it takes. Use your normal shock dose as the baseline and multiply:
| Severity | What it looks like | Shock dose |
|---|---|---|
| Light | Faint green or teal tint, water still clear | About 2× a normal dose |
| Moderate | Green and cloudy, bottom hard to see | About 3× a normal dose |
| Severe | Deep green, can't see the floor at all | About 4× — expect to repeat |
The finish line to watch for: keep chlorine high until the water turns cloudy gray or milky white. That color change means the algae is dead — now it's a filtering-and-vacuuming job, not a chlorine job.
Note on other algae
Dark spots that cling to walls (black algae) or a dusty yellow film (mustard algae) are more stubborn than green and need heavier, more targeted treatment. If brushing and shocking don't clear it, message our pool techs for a tailored plan.
How long to clear a green pool?
- Light green: often clear in 1 to 2 days.
- Moderate: usually 2 to 3 days with continuous filtration.
- Severe: 3 to 5 days, with repeat shocking and vacuuming as the dead algae settles.
As with cloudy water, the filter does the heavy lifting once the algae is dead. For that side of the job, see our guide on clearing cloudy pool water — a green pool almost always passes through a cloudy stage on its way back to clear.
Keep algae from coming back
Algae only returns when chlorine slips. Prevent the next bloom by keeping free chlorine at 1–3 ppm at all times (never let it hit zero), keeping cyanuric acid in the 30–50 ppm range so your chlorine stays effective, brushing weekly, running the pump 8–12 hours a day, and shocking every week or two. Steady sanitizing with 3-inch chlorine tablets is the simplest way to hold that line, and our weekly pool maintenance schedule keeps every step on track so green never gets a second chance.
Frequently asked questions
Will pool shock alone kill algae, or do I also need algaecide?
A heavy dose of shock is the main killer — it raises chlorine high enough to destroy an active bloom. Algaecide is a backup and a preventive: add it after chlorine returns to normal to help stop algae coming back. For a full green pool, lead with shock.
Why did my pool turn green overnight?
Green almost always means free chlorine hit zero and algae took off. A heat wave, heavy rain, a broken pump, or a missed week can bottom out chlorine, and algae blooms from clear to green in 24 to 48 hours once nothing is stopping it.
How much shock do I need for a green pool?
Roughly 2 to 4 times a normal dose by severity — double for a light tint, triple for green and cloudy, quadruple for deep green. Keep chlorine high until the water turns cloudy gray or white, which means the algae is dead.
Can I swim in a green pool?
No. Green water has poor visibility and can harbor bacteria alongside the algae. Wait until it's fully clear and free chlorine settles back to 1–3 ppm before swimming.
Should I just drain a green pool instead of treating it?
Usually not. Draining is expensive, wastes water, and can damage a shell or liner, so it's a last resort for extreme cases. Brushing, shocking, and filtering clears the vast majority of blooms far more safely.
This guide is general educational information. Treatment needs vary by pool size, algae type, surface, and local conditions — always follow the directions on each product label and test your water before dosing.
