Your pool turned green almost overnight, or it went milky right after you shocked it. Green and cloudy pool water nearly always comes down to three things: too little working chlorine, a filter that is not keeping up, or metals in the water. This page explains what keeps water clear, how to tell algae from metals, the numbers to aim for, and when to call a pro.
What keeps pool water clear?
Clear water needs working chlorine, a pump and filter that keep the water moving, and balanced chemistry. LaMotte, a test kit maker, notes that by the time you can see algae, there are already billions of algae cells in the pool.
- Free chlorine kills. Free chlorine is the chlorine still able to kill germs and algae, measured in ppm (parts per million). LaMotte says the best way to avoid algae is to keep at least 2.0 ppm in the water at all times.
- pH lets chlorine work. pH tells you how acidic or basic the water is. CDC notes that chlorine kills germs less well as pH goes up, especially above 8.0.
- Stabilizer shields chlorine from the sun. Cyanuric acid (CYA) slows the sun from burning off chlorine in outdoor pools. Too much of it makes chlorine slow to act.
- The pump turns the water over. Turnover is the time it takes for all the pool water to pass through the filter once. LaMotte suggests a full turnover at least every six hours in busy pools.
- The filter catches particles. Dead algae, dust and other fine bits stay in the water as haze until the filter traps them.
What causes green or cloudy pool water, and what are the signs?
| Cause | What it does to the water | How to tell |
|---|---|---|
| Low free chlorine | Lets algae grow, so water goes hazy, then green | Free chlorine near zero, slimy walls, murky green color |
| High pH | Weakens chlorine and can turn water cloudy | pH test reads above 7.8 |
| Too much CYA | Slows chlorine down, so algae can grow with chlorine in the water | Normal chlorine reading, algae anyway, CYA well above 50 ppm |
| Poor filtration | Fine particles stay in the water | Cloudy but chlorine and pH are fine; short pump run or dirty filter |
| Copper | Turns water blue-green or aquamarine, often after a big chlorine dose | Water is see-through but tinted; blue-green stains |
| Iron or manganese | Turns water green, brown, red or black, often right after shocking or filling | Clear, tinted water; brown or rusty stains; fill water from a well |
| High calcium, alkalinity or TDS | Minerals come out of the water as haze and scale | Calcium over 400 ppm, white crust at the tile line |
LaMotte gives a simple rule: water tinted by metals stays see-through in the early stages, while murky, turbid green water is usually algae.
What should I check first when my pool turns green or cloudy?
- Pump and filter. Make sure the pump runs long enough each day and the filter is clean. If the filter is the problem, see our pool filter problems hub.
- Free chlorine and pH. Test both. If free chlorine is near zero, you have algae food and no defense. A yellow pH test means pH is at or below 6.8, the bottom of most phenol red scales. A purple pH test can mean chlorine is above 10 ppm and is throwing off the reading.
- Murky or see-through. Look down into the deep end. Murky green points to algae. Clear but tinted water points to metals.
- Recent shock or fresh fill. If the color showed up right after you shocked or topped off from a well, test for copper and iron before adding more chlorine. LaMotte warns that a big chlorine dose can make metals color the water at once.
- CYA. If chlorine reads fine and algae keeps coming back, test stabilizer. Our pool chemistry hub covers how CYA and chlorine work together.
- Phosphate and nitrate. These feed algae. A pool store or a test kit can check them.
Safety first. Never mix pool chemicals with each other; CDC warns they can injure people when mixed, and chlorine plus acid is a classic mistake. If a label says to dissolve a product first, add the chemical to water, never water to the chemical. Keep everyone out of the pool while you shock and until the chlorine drops back down. Do not let anyone swim in water so cloudy you cannot see the bottom of the deep end. Store chemicals dry, cool and locked away from kids and pets.
Green and cloudy pool water numbers to know
These numbers come from CDC and from the test kit makers Taylor Technologies and LaMotte.
| Item | Number | Source |
|---|---|---|
| Free chlorine, minimum | At least 1 ppm; at least 2 ppm if you use stabilizer | CDC |
| Free chlorine to keep algae away | At least 2.0 ppm at all times | LaMotte |
| pH | 7.0 to 7.8; 7.4 to 7.6 is ideal | CDC; LaMotte |
| Shock level for normal shocking | Raise free chlorine to 10 ppm | LaMotte |
| Hazy, slightly green water from algae | Superchlorinate to 30 ppm and brush the surfaces | LaMotte |
| Chlorine test limit | Above 10 ppm, the test can bleach out and read low | CDC; Taylor |
| Copper and iron | Ideal is 0 ppm | LaMotte |
| Algae food | Phosphate under 100 ppb; nitrate under 10 ppm | LaMotte |
Liquid chlorine vs cal-hypo: which shock clears a green pool?
To kill algae you need a chlorine shock, because algae dies when free chlorine goes high and stays high. Non-chlorine shock, the persulfate kind, breaks down used-up chlorine but does not raise free chlorine, so LaMotte notes it does not replace chlorine. It will not clear a green pool.
Liquid chlorine (sodium hypochlorite) mixes in at once and leaves nothing behind. It is strongly basic, so it nudges pH up, and you will likely need acid afterward. Cal-hypo (calcium hypochlorite) granules are also basic, and every dose adds calcium. In a pool with hard water, that extra calcium can add to haze and scale. Dichlor and trichlor contain stabilizer, so big doses raise CYA: Taylor says each pound of trichlor in 10,000 gallons adds 7 ppm of CYA. For a green pool, we reach for liquid chlorine first in most cases, because it fixes the chlorine without piling on calcium or stabilizer.
Salt pools follow the same rules; if yours went green, see our salt water pool problems hub.
When should you call a pool or spa pro?
- Still green after several days. If chlorine will not stay up while you brush and filter, something is eating it, and a full lab test helps.
- Metal tint or stains. LaMotte notes that stains left too long in plaster pools may need the pool drained and acid washed.
- Black algae spots that return. LaMotte’s treatment calls for hard brushing, spot treating and a 30 ppm shock, and it can take more than one round.
- Filter or pump trouble. High pressure, leaks, no flow, or filter media blowing back into the pool.
- You think you need to drain. Draining can damage some pools, so let a pro decide.
Pool pros bill by the visit or by the hour, so testing and noting your numbers first saves time on the clock.
Common questions
Will shock clear a cloudy pool?
Shock clears cloudiness caused by algae or used-up chlorine. It will not fix haze from a dirty filter, short pump runs or high calcium. After shocking, run the filter around the clock and clean it as the pressure rises, because dead algae is what keeps the water cloudy.
Why is my pool cloudy but the chlorine is good?
Good chlorine with cloudy water usually points to filtration or water balance. Check the filter and pump run time, then test pH, alkalinity and calcium; LaMotte lists high pH, high calcium and high TDS as causes of cloudy water.
Will a cloudy pool clear on its own?
Rarely. Haze clears only when the filter catches it, and that will not happen if chlorine is low or the water is out of balance. Fix the cause, then run the filter; LaMotte suggests running it for 24 hours.
Can you fix a green pool without draining?
Yes, in most cases. Test and correct pH, raise free chlorine to shock level, brush every day and keep the filter running and clean. Draining only makes sense when something cannot be fixed with chemicals, such as very high CYA or heavy metal staining, and a pro should make that call.
Why is my pool green but the chlorine is high?
If the water is green but see-through, metals are the likely cause: iron can turn water green or brown, and copper can turn it aquamarine after a big chlorine dose. If it is murky green with high chlorine, test CYA, because too much stabilizer slows chlorine down. Also check the reading itself, since chlorine above 10 ppm can bleach the test.
How long does it take to clear a green and cloudy pool?
There is no set time; it depends on how much algae there is and how well the filter keeps up. LaMotte notes that daily brushing and several treatments may be needed, so plan on days, not hours.
Is it safe to swim in green pool water?
No. Algae means chlorine is too low to protect you from germs, and murky water hides swimmers who get into trouble. Wait until the water is clear, free chlorine is in range and pH is between 7.0 and 7.8. After a shock, LaMotte gives 4 ppm as a safe level to swim again.