Why is my pool green after I opened it?
Green water in spring is almost always algae that took hold under the cover or in the first days after opening, because the chlorine was low, the pH was off or the water was not circulating. Test and correct pH and alkalinity first, brush every surface, run the pump around the clock, then raise chlorine as the product label directs.
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By Kasra Taheri, Founder, LoclisPublished
Researched and drafted with AI, then checked claim by claim against the sources listed below before it was published. Prices and rules change; the dates beside each source say when it was last checked.
Step by step
Read when to stop and call a pro, further down this page, before you start.
Step 1: Clean out the baskets
Empty the skimmer and pump baskets and skim leaves off the surface so water can move freely.
Step 2: Run the pump continuously
Run the pump around the clock until the water clears, so the whole pool is turned over and filtered.
Step 3: Test the water
Take a sample from 12 to 18 inches below the surface, away from corners and return lines, and test chlorine, pH and alkalinity.[1]
Step 4: Correct pH and alkalinity
Bring pH and total alkalinity into range first, adding each product separately and following its label.[1]
Step 5: Brush every surface
Brush walls, steps, corners and behind ladders so algae is loosened into the water and reaches the filter.
Step 6: Raise the chlorine
Add chlorine as the product label directs to superchlorinate; on a pool with a lot of stabilizer, use a non-chlorine product instead.[1]
Step 7: Check the filter daily
Check the filter gauge each day and clean or backwash the filter as its manual says.
Step 8: Retest and repeat
Retest the water each day and repeat brushing and chlorine until the water is clear and the readings hold.
Algae, and chlorine that was not there to stop it
A pool that is green after opening has algae in it. It usually started under the cover or in the first days after you took it off, when the water was warming, the chlorine was low and the pump had not yet turned the water over.
Chlorine acts as an oxidizer that chemically burns away compounds such as bacteria, fungi, viruses and algae.[1] When the free chlorine is spent, algae spores that were always in the water multiply. Virginia's water quality standard for pools keeps free chlorine between 1.0 and 3.0 ppm and pH between 7.2 and 7.8.[2] That standard governs regulated pools such as public ones, but it is a sensible target at home too.
A cover alone does not keep the water clear. The Portsmouth guide's closing checklist notes that a pool left uncovered for winter takes more work to condition on opening day.[1] Even a covered pool can bloom if the chlorine was gone before closing, or if the cover let water and debris collect on top.
Why it happens after opening in particular
Low pH. A pH below 7.0 will also encourage algae to grow.[1]
Algae pushing pH up. An algae bloom can itself push the pH of the water up,[1] so a green pool with a high reading may have the bloom as both cause and effect.
Low alkalinity. Total alkalinity that is too low can cause rapid changes in pH, corrosion, staining and possibly green water.[1]
Poor circulation. Dead spots, where circulation is poor, can leave water poorly disinfected and may promote the growth of bacteria and algae.[1] Behind ladders, in corners and below steps are the usual places.
Late start-up. A pump run for only a few hours a day, or switched on late, leaves a cold, still pool a head start. If you are still opening it, follow how to open a pool in spring and get the pump running before the water warms.
How to clear it
Start with circulation. Empty the baskets and run the pump around the clock until the water clears.
Test next. Take water samples from 12 to 18 inches below the surface, and not from corners or directly in front of return lines.[1] Pool test kits are not 100% accurate, so use fresh reagents and replace them at the start of each season.[1] A kit left in the garage all winter is a common reason for a wrong reading. If yours is old, take a sample to a pool supply store.
Then correct pH and alkalinity before chlorine, and brush every surface. Algae stuck to a wall is protected from chlorine and never reaches the filter. For adjusting the numbers, see how to balance pool chemicals.
After that, raise the chlorine. Super chlorination means raising the free chlorine residual of the pool water to 5 to 10 ppm,[1] and the product label tells you how much of your product gets there. Never mix anything with chlorine except water,[1] and add each product separately. If your pool carries a lot of stabilizer from tablets, note that on stabilized pools, the guide recommends non-chlorine products for superchlorination, because chlorine can become trapped in the water when cyanuric acid builds up.[1]
Keep the filter working while the water clears. Check its gauge every day and clean or backwash it as its manual says; a cartridge filter may need cleaning several times. As the green turns to cloud, clearing cloudy pool water is the second half of the job.
When it is clear
Do not trust looks alone, because clear water doesn't necessarily mean that the pool is sanitary,[1] so retest and let the readings, not the colour, tell you it is ready. Then settle into a routine; how often to test pool water sets one out. For the usual compare-the-options question on who should do the opening work, see open and close it yourself or hire a pool company.
Stop and call a pro if
- The pool is still green after a week with chlorine and pH in range and the filter clean. The cause is likely the filter, the circulation or a stabilizer level far above range.
- The pump will not prime, leaks or grinds. Pump and plumbing repair is a job for a pool professional.
- A light, automation panel, heater or buzzing motor is involved. Do not open or rewire electrical or gas equipment yourself.
- You cannot see the main drain from the deck. Keep everyone out of the water until it is treated.
Getting help in Virginia
Loclis lists licensed local businesses that open, clean and service pools. Browse pool and spa pros or read more pool and spa answers. When you are ready, describe the job and local pros quote it.
Common questions
- Is green pool water safe to swim in?
- Stay out until it is clear. Green water means the chlorine has not kept up, and the Portsmouth Health Department's operator guide notes that clear water does not necessarily mean a pool is sanitary, so the reverse is a clear warning. Treat it, retest, and swim once the readings are in range and you can see the bottom.
- Why did the pool go green when it sat under a cover all winter?
- A cover keeps leaves out but does not keep a pool sanitary, and chlorine fades over months. The Portsmouth guide notes that a pool left uncovered for winter takes more work to condition for opening day, and a covered pool with a spent residual can still bloom once the weather warms.
- Should I add chlorine or fix pH first?
- Fix pH and alkalinity first. An algae bloom can itself push the pH up, so a green pool is often out of range, and the readings tell you what to correct. Then add chlorine as the label directs.
- How long does it take to clear a green pool?
- Often a few days with the pump running constantly and daily brushing, longer for a heavy bloom. Dead algae turns the water cloudy as it dies, and the filter has to catch it, so keep the filter clean while the water clears.
- Can I just drain the pool and start again?
- Usually not worth it. A bloom is a chemistry and circulation problem, and refilling costs water and time without fixing either. Draining some pool types can also damage the shell, so ask a pool company before emptying one.
Sources
- [1] Pool Operation and Maintenance Guide (compiled 2018), Portsmouth Health Department, Virginia Department of Health. Checked October 28, 2026.
- [2] 12VAC5-462-290. Water quality standards, Virginia Administrative Code, Virginia General Assembly. Checked October 28, 2026.
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