You walk outside and your pool looks like a swamp. The water is green, the walls feel slimy, and you can barely see the bottom. Algae has taken over.
Do not panic. Green pool water is one of the most common pool problems and it is completely fixable. It takes some effort and a few days of work, but you can go from swamp to crystal clear without draining and refilling. Here is exactly how to do it.
Why Your Pool Turned Green
Green water is almost always caused by algae. Algae spores are everywhere in the environment (wind, rain, swimsuits, pool toys) and they will bloom whenever conditions are right. Here are the conditions that let algae win.
Low Chlorine
This is the number one cause. Chlorine is what kills algae. When free chlorine drops below 1 ppm, algae can start growing. Below 0.5 ppm, it can explode overnight. A few common reasons chlorine drops too low: you forgot to add it, your chlorinator ran out of tablets, the salt cell needs cleaning, or you shocked but the chlorine demand was higher than expected.
Poor Circulation
Dead spots in your pool where water does not circulate well are where algae starts. Behind ladders, in corners, on steps, and around the main drain are common trouble spots. If your pump is not running enough hours per day, or if your return jets are not properly aimed, circulation suffers.
High pH
When pH climbs above 7.6, chlorine becomes significantly less effective. At 8.0, chlorine is only about 20 percent as effective as it is at 7.2. So even if your chlorine test reads 2 ppm, the actual killing power is much lower at high pH.
Warm Water
Algae loves warm water. As temperatures climb above 80 degrees, algae growth accelerates dramatically. This is why mid-summer is peak algae season.
Types of Algae
Green algae is the most common and the easiest to kill. It floats in the water and coats surfaces, turning everything green. This is what most pool owners deal with.
Yellow (mustard) algae clings to walls and the floor, usually in shady spots. It looks like sand or pollen and brushes off easily but comes right back. It is more chlorine-resistant than green algae.
Black algae forms dark spots or patches on pool surfaces, especially plaster and concrete. It has deep roots and a protective outer layer that makes it the hardest algae to kill. Black algae almost always requires aggressive brushing with a steel brush in addition to heavy shocking.
Step-by-Step: How to Fix Green Pool Water
Step 1: Test the Water
Before you do anything, test your water with a reliable pool test kit. You need to know your starting point for free chlorine, pH, and alkalinity.
If the water is dark green and you cannot see the bottom, your chlorine is almost certainly at zero. pH may be high, low, or anywhere depending on what has been happening in the pool. Write down all the numbers.
Step 2: Adjust pH First
Get pH between 7.0 and 7.2. Yes, slightly lower than the normal target of 7.4 to 7.6. Chlorine is most effective at lower pH, and you need maximum killing power right now.
If pH is high, add muriatic acid. If it is low, leave it alone for now. You will bring it back to normal after the algae is dead.
Step 3: Brush Everything
Before you shock, brush every surface in the pool. Walls, floor, steps, behind the ladder, inside the skimmer throat, around light niches, everywhere. Brushing breaks up the algae and removes its protective biofilm so the chlorine can reach and kill it.
Use a stainless steel brush on concrete and plaster pools. Use a nylon brush on vinyl and fiberglass to avoid scratching. This step is not optional. Shocking without brushing is like spraying weeds without pulling them first.
Step 4: Shock Hard
This is not a maintenance shock. You need to hit the pool with a massive dose of chlorine to overwhelm the algae.
For green water where you can still see the bottom, use 2 pounds of calcium hypochlorite (cal-hypo) shock per 10,000 gallons. For dark green water where you cannot see the bottom, use 3 to 4 pounds per 10,000 gallons.
Liquid chlorine (sodium hypochlorite) works too. Use 2 to 4 gallons per 10,000 gallons depending on severity.
Add the shock at dusk. Sunlight destroys chlorine, and you want the maximum contact time overnight.
Do not use trichlor shock for this purpose. It contains cyanuric acid (stabilizer) and will raise your stabilizer level, which makes future shocking less effective. Cal-hypo or liquid chlorine are the right tools for heavy shocking.
Step 5: Run the Pump 24/7
After shocking, run your pump and filter continuously until the water is clear. This means 24 hours a day, no exceptions. The filter needs to process the water multiple times to remove dead algae.
If you have a sand filter, backwash it when the pressure gauge rises 8 to 10 psi above clean baseline. You may need to backwash every 8 to 12 hours during heavy algae treatment.
If you have a cartridge filter, clean the cartridge every 12 to 24 hours. It will clog fast as it catches dead algae.
If you have a DE filter, backwash and recharge with fresh DE when pressure rises.
Step 6: Brush Again
The next morning after shocking, brush everything again. Dead algae settles on surfaces and the filter needs help picking it up. Brushing also exposes any remaining live algae to the chlorine still in the water.
Step 7: Vacuum Dead Algae
As the water starts to clear (usually turning from green to cloudy blue-white), you will see dead algae settled on the floor. Vacuum it out.
If you have a lot of dead algae, vacuum to waste rather than through the filter. This sends the water straight out of the pool instead of through the filter, preventing the filter from clogging. You will lose some water, but you can refill after.
Step 8: Retest and Rebalance
After 24 to 48 hours of continuous filtration, test the water again. If free chlorine has dropped back to zero, you need to shock again. The algae ate all the chlorine and there may be more to kill.
Once chlorine holds at 1 to 3 ppm after 24 hours, the algae is dead. Now rebalance your water chemistry:
- pH: 7.4 to 7.6
- Alkalinity: 80 to 120 ppm
- Cyanuric acid: 30 to 50 ppm
- Calcium hardness: 200 to 400 ppm
The pool should go from green to cloudy to clear over 2 to 5 days depending on how bad the bloom was and how well your filter works. Check our pool pump troubleshooting guide if your pump is not performing well during this process.
Prevention: Keeping Algae From Coming Back
Killing algae is satisfying but preventing it is smarter. Here is what keeps your pool algae-free.
Maintain chlorine levels. Keep free chlorine between 1 and 3 ppm at all times. Test at least twice a week during summer. A saltwater system or automatic chlorinator makes this easier.
Run the pump enough. Your pool water should turn over at least once per day. For most residential pools, that means 8 to 12 hours of pump runtime. In peak summer, lean toward more.
Brush weekly. A weekly brushing prevents algae from getting a foothold on surfaces, especially in spots with poor circulation.
Keep pH in range. Maintain pH between 7.2 and 7.6 so your chlorine stays effective. Check out how to lower pool chlorine and our guide on pH management for the full picture.
Shock regularly. A maintenance shock every 1 to 2 weeks during swimming season breaks down chloramines and kills any algae spores before they can bloom.
Stay on schedule. A consistent pool maintenance schedule is the best defense against algae. It is much easier to spend 30 minutes a week on maintenance than to spend a week fixing a green pool.
Clean the filter. A dirty filter means poor circulation, which means dead spots where algae grows. Clean or backwash according to your filter type and schedule.
When to Call a Professional
Most green pool situations are DIY-fixable. But there are a few cases where a pro might be the better call:
- Black algae that keeps coming back despite repeated treatment
- Pool has been green for weeks or months and has significant debris accumulation
- You suspect equipment problems (pump not working, filter damaged, possible leaks)
- The pool water is so dark you cannot see 6 inches below the surface and you are not comfortable handling large quantities of chemicals
A pool service company can typically clear a green pool in 3 to 5 days and charge $200 to $600 depending on severity and your location. Sometimes that is money well spent if you want it dealt with quickly. If you are opening a pool that sat green all winter, our how to open your pool for summer guide covers the full spring startup process.
Frequently Asked Questions
How long does it take to fix a green pool?
Light green water (you can still see the bottom) typically clears in 1 to 2 days with proper shocking and filtration. Dark green water where you cannot see the bottom usually takes 3 to 5 days. Extremely neglected pools that have been green for months may take a week or more, especially if you need to shock multiple times.
Can I swim in a green pool?
No. Green water means algae is present, and algae creates an environment where harmful bacteria (including E. coli) can thrive. The water also has insufficient chlorine to provide safe sanitation. Wait until the water is clear and chlorine levels are between 1 and 3 ppm before swimming.
Will algaecide fix a green pool?
Algaecide alone will not fix a green pool. Algaecide is a preventive tool, not a cure. It works best as a supplement after you have killed the existing algae with chlorine shock. Adding algaecide to a green pool without shocking first is a waste of money. Shock first, then add a maintenance dose of algaecide after the pool clears to help prevent future blooms.
Why did my pool turn green overnight?
A sudden green bloom usually means chlorine dropped to zero or near-zero while conditions were perfect for algae growth (warm water, available nutrients, poor circulation). This can happen after a heavy rain dilutes your chemicals, after a pool party with many swimmers that used up the chlorine, or simply because you ran out of tablets and did not notice for a few days.
Do I need to drain a green pool to fix it?
Almost never. Draining creates risks to the pool structure and is usually unnecessary. The shock-and-filter method described above will clear even severely green pools. The only situation where draining might make sense is if the water has extremely high levels of cyanuric acid (above 100 ppm) that prevent chlorine from working effectively. In that case, a partial drain and refill (no more than one-third of the water) can lower stabilizer to a workable level.