Green pool water: what to do
Figures checked 11/9/2026
Keep everyone out and ask a pool professional (for example a pool store water test) for a test and a plan. Green, black or very cloudy water needs a professional test and plan, so our calculators give no amounts for it.
Why we give no doses for green water
Our dose charts are for water you can see through. CDC's pool inspection guide includes the line Drain at the bottom of the deep end is visible.
When you can't see the drain at the bottom of the deep end, or the water is green or black, every chemical calculator on this site asks how the water looks, stops, and points you to a pool professional instead of guessing.
Green, but the chlorine reads high or zero
A home test can be wrong at high chlorine. CDC says If the chlorine concentration of the water sampled from the pool or hot tub is more than 10 ppm, the test might partially or totally bleach out, resulting in a false low or 0 ppm chlorine result.
That is one more reason to have the water tested by a pool professional before adding anything.
The SLAM method
SLAM is a method from the Trouble Free Pool forum that keeps chlorine above the levels on product labels. Our calculators don't use it. Most chlorine labels we checked say nobody should swim above 4 ppm.
What we don't cover
We don't list the causes of green water or give a step-by-step recovery plan: the official sources we use don't give a checked one, and a plan depends on a proper water test. Phosphates come up often on this subject; we have no checked figures on them yet.
Questions people ask
Why is my pool green?
We don't list causes, because the official sources we use don't give a checked list. Keep everyone out and ask a pool professional for a water test and a plan.
Can I shock a green pool?
Our shock calculator gives no amount for green, black or very cloudy water: that water needs a professional test and plan. Ask a pool professional before you add anything.
My pool is green but the chlorine is high. Why?
We can't tell you why from a reading. Home tests can mislead when chlorine is high: CDC says a test can bleach out above 10 ppm and read falsely low or zero. Ask a pool professional for a test.
Can I clear a green pool overnight?
We can't give a time. Keep everyone out and follow the plan a pool professional gives you after a water test.
Can I swim in a green pool?
No. Keep everyone out and ask a pool professional.
The charts behind this page
Show all 3 figures and where each one comes from
Each figure this page and its calculator use. Open one for the source's own words, the other sources we checked it against, and the checks it has been through.
Free chlorine above which chlorine labels say do not swim (re-entry level): 4 ppm Other source
EPA-accepted label: B's Pool Supply Sodium Hypochlorite Solution 12.5% (EPA Reg. 55487-20001) Other source says:
SWIMMING POOL WATER DISINFECTION – Do not reenter pool with a chlorine level above 4 ppm for risk of bodily harm.
Source page (opens in a new browser tab)Also checked: EPA-accepted label: BioGuard Master Calcium Hypochlorite 67% (Bio-Lab, EPA Reg. 5185-238) Other source says:
Reentry into treated swimming pool is prohibited above levels of 4 ppm chlorine due to risk of bodily injury.
Source page (opens in a new browser tab)Also checked: EPA-accepted label: PDQ ChlorSan 8% Sodium Hypochlorite (EPA Reg. 58880-2) Other source says:
Reentry into treated pools is prohibited above levels of 4.0 ppm due to risk of bodily harm.
Source page (opens in a new browser tab)Also checked: HTH Pool Care Shock product page Other source says:
Do not enter the pool until free available chlorine levels return to 1–4 ppm.
Source page (opens in a new browser tab)Also checked: EPA-accepted label: Tarry trichlor 8 oz tablets/sticks (Qualco, EPA Reg. 3525-120) Other source says:
Re-entry into treated swimming pools/spas is prohibited above levels of 3.0 ppm.
Source page (opens in a new browser tab)Also checked: EPA-accepted label: Tarry trichlor 8 oz tablets/sticks (Qualco, EPA Reg. 3525-120) Other source says:
Do not enter pool until the chlorine residual drops to 2.0 ppm (parts per million).
Source page (opens in a new browser tab)Also checked: CDC Model Aquatic Health Code, 5th edition (2024), Code Official says:
Maximum DPD-FC concentrations shall not exceed limits specified on the product label.
Source page (opens in a new browser tab)Checked against the source on 11/9/2026. Re-opened by a second checker on 11/9/2026, who matched the source's words to this figure. Looked up again on 11/10/2026 by a checker who couldn't see our figure: it came back with the same figure.
Free chlorine reading above which the test may bleach out (false low): 10 ppm Official
CDC: Home pool and hot tub water treatment and testing Official says:
If the chlorine concentration of the water sampled from the pool or hot tub is more than 10 ppm, the test might partially or totally bleach out, resulting in a false low or 0 ppm chlorine result.
Source page (opens in a new browser tab)Checked against the source on 11/9/2026. Re-opened by a second checker on 11/9/2026, who matched the source's words to this figure.
You should be able to see the drain at the bottom of the deep end: the drain at the bottom of the deep end can be seen Official
CDC healthy swimming infographic: inspection Official says:
Drain at the bottom of the deep end is visible.
Source page (opens in a new browser tab)Also checked: Licking County Health Department (Ohio): Pool and Spa Water Chemistry Adjustment Guide Official says:
Clarity Deepest point of pool visible from deck
Source page (opens in a new browser tab)Checked against the source on 11/9/2026. Re-opened by a second checker on 11/9/2026, who matched the source's words to this figure.