Fixing green or cloudy pool water

Before you add anything, look closely. Different colors and kinds of haze have different causes, and the fix depends on which one you have.

• Green and cloudy, slimy walls: algae. This is the common one.
• Clear but tinted green, blue-green or brown, often right after adding shock: metals such as copper or iron coming out of solution. Well water and old copper heat exchangers are the usual sources.
• Milky or dull white haze, walls not slimy: chemistry or filtration.
• Yellow film on the surface, yellow dust on the steps in late spring: pollen. It can look like mustard algae, but it skims off and does not cling.
• Dark spots that do not brush away easily: stubborn algae or staining, worth a professional look.

Test the water before you decide. You need free chlorine, pH, total alkalinity and, if you use tablets, cyanuric acid. A pool store can run a fuller test if your kit only covers the basics.

Algae is always around. It rides in on wind, rain, pollen and swimsuits. Chlorine keeps it from growing. Anything that lets free chlorine drop for a day or two in warm water can let it bloom. In Northeast Ohio the usual triggers are:

• A late opening. Water under a winter cover warms in May and June, the chlorine left from closing is long gone, and the cover comes off to a swamp.
• A heat wave. Warm water and strong sun burn off chlorine faster than the usual routine replaces it.
• A big thunderstorm. Rain dilutes chlorine, shifts pH and washes in organic material from the lawn and trees.
• A pool party. Many swimmers use up chlorine quickly.
• A pump that stopped. A tripped breaker or a timer reset after a power cut leaves the water still.
• Too much stabilizer. The CDC notes that disinfection takes longer in the presence of cyanuric acid. Tablets add it steadily through the season.

1. Get the filter running and clean. Empty the skimmer and pump baskets. Backwash or clean the filter so it starts fresh. Run the pump continuously until the job is done.
2. Clear out debris. Net leaves and sticks off the floor. Rotting leaves eat chlorine you need for the algae.
3. Balance pH first. Bring it into the 7.2 to 7.8 range the CDC recommends, toward the low end. Pentair notes that chlorine does not work as well when pH is high, so shocking a high-pH pool wastes product.
4. Brush every surface. Walls, floor, steps, corners, behind ladders and in the skimmer throats. Brushing breaks the algae's grip and exposes it to chlorine.
5. Shock heavily. Add enough shock to raise free chlorine well above your normal level, following the product label for a green pool. Add it in the evening so the sun does not burn it off. With a lot of stabilizer in the water, you need a higher chlorine level to get the same effect.
6. Keep the filter working. The filter will clog as it catches dead algae. For a DE filter like the Pentair FNS Plus, Pentair says to backwash when pressure reads about 10 psi over the clean starting pressure, then recharge with DE. Expect to do this more than once.
7. Test and repeat. Test chlorine in the morning. If it has dropped a lot overnight, algae is still using it up, so brush and shock again. The water will usually turn cloudy blue-gray as the algae dies. That is progress.
8. Vacuum. When the dead algae settles, vacuum it up. With a heavy layer, vacuuming to waste, bypassing the filter, avoids blowing it back into the pool.
9. Rebalance. Once the water is clear and chlorine holds overnight, bring alkalinity, pH and chlorine back to normal levels.

A light tint clears in a day or two. A swamp after a late opening can take several days and several rounds of shock and filter cleaning.

Cloudy water without slime usually has one of three causes.

Chemistry. Pentair's water guide points to high pH, high alkalinity and very hard water, all of which can cloud the water and cause scale. Test and correct alkalinity, then pH. If calcium hardness is very high, partly draining and refilling may be the only fix. Check with your builder first, especially with a fiberglass pool.

Filtration. A filter that is dirty, undersized for the job, or not coated with enough DE lets fine particles through. So does a pump schedule that is too short. Clean the filter, recharge the DE if you have one, and run the pump longer for a couple of days.

Early algae or heavy use. A faint haze after a busy weekend often means chlorine has run low. Test and bring free chlorine up. If it keeps falling, treat it as the start of an algae problem.

Clarifiers can help a filter catch very fine particles, but they only help if the chemistry and filter are right first.

If the water turned green, teal or brown right after shocking and is still clear rather than cloudy, you are probably looking at metals. Shock oxidizes dissolved copper or iron and makes them visible. This is common when a pool is topped up from a well, which is how plenty of homes in rural Portage, Geauga and Trumbull counties get their water. More shock makes it worse.

Stop adding chlorine, get a metals test done, and use a metal sequestrant made for pools. A pre-filter on the hose when you top up from a well helps prevent it next time.

Most green pools are the common free-floating green algae, and the steps above handle it. Two other kinds are more stubborn. Yellow or mustard algae shows up as a dusty yellow-brown patch, usually on the shady side of the pool, and brushes off easily but comes back within a day or two. Black algae forms small dark spots with a hard surface, mostly in plaster and concrete pools, and has roots that grow into the surface. Both need harder brushing, a stiff brush on concrete, more rounds of shock and patience. Treat the brushes, toys and swimsuits that were in the water too, since they can bring it back.

Yes. A salt system makes chlorine, but only as fast as the cell is set to produce and only while the pump is running. In a heat wave or after a storm, a salt cell can fall behind like anyone adding chlorine by hand. A scaled-up cell makes less chlorine than its setting says. If a salt pool turns green, shock it with chlorine the same way, then clean the cell and raise the output.

No. Green water means chlorine has been too low to kill germs, and you cannot see the drain, which is a drowning risk in itself. While you shock, chlorine will be far above normal swimming levels. Wait until the water is clear enough to see the bottom and free chlorine and pH are back in the CDC's ranges before anyone swims.

Handle the chemicals carefully. The CDC's pool chemical safety guidance is to read the label before each use, wear goggles and gloves, open one container at a time and never mix chlorine products with acid or with each other, because that can create toxic gas. Add chemicals to water, not water to chemicals, and store them dry and apart.

• Open the pool early enough, before warm weather, and keep chlorine up from day one.
• Test more often in heat waves and after storms.
• Keep pH in range so chlorine works.
• Brush weekly, including the steps and corners where water moves slowly.
• Watch the filter gauge and clean it when it tells you to.
• If you use tablets, test cyanuric acid every few weeks.
• Check the pump is running after any power cut.

Pool type matters too. A fiberglass pool's gel coat is smooth and non-porous, which gives algae much less to hang on to than plaster. Circulation matters just as much. A variable-speed pump on a long, slow schedule keeps water moving through the filter for more hours of the day.

Call someone if the pool is still green after several days of doing everything right, if chlorine vanishes overnight no matter how much you add, if you suspect metals, if the filter pressure will not come down after cleaning, or if the pump is not moving water properly. Those point to problems beyond chemistry, such as a failing filter grid, a pump issue or a very high stabilizer level that needs partial water replacement.

We handle green pool cleanups as part of our pool service, along with weekly cleaning, water testing and chemical balancing, repairs and equipment checks.