How to Fix a Green Pool: A Step-by-Step Recovery Plan
Few things deflate a pool owner faster than lifting the cover, or glancing out the window, and seeing green water. The good news is that most green pools can be recovered without draining, without a service call, and without buying every product on the store shelf. The key is to work in order: test, fix circulation, brush, sanitize, filter, and then prevent. Skipping steps is what turns a three-day job into a three-week one.
Why pools turn green
Green water is almost always algae. Algae spores are constantly blown or carried into every pool. They only take over when the sanitizer level drops too low to kill them, usually because of a combination of heat, sunlight, heavy use, rain, poor circulation, or a filter that is not doing its job. High pH makes chlorine less effective, and low or missing cyanuric acid in an outdoor pool lets sunlight burn chlorine off within hours. In other words, green water is a symptom. The real problem is that chlorine lost the race.
Step one: test before you dose
Use a reliable test kit to check free chlorine, pH, total alkalinity, and cyanuric acid (stabilizer). Write the numbers down. If you cannot see the bottom of the pool, the water is cloudy enough that you also need to treat it as a swimming hazard: keep everyone out until you can clearly see the main drain.
Testing first prevents the most common mistake, which is pouring in shock blindly. If cyanuric acid is very high, for example, a normal dose of chlorine will barely work, and you will need far more or need to partially drain to bring stabilizer down.
Step two: get the water moving
Clean the skimmer and pump baskets, then check the filter. Run the pump continuously, around the clock, until the water is clear. Algae thrive in still water, and the filter cannot remove what the pump never sends to it. Point return jets downward and in a circular pattern to help move water across the floor.
If the water level is low, top it up so the skimmer does not suck air. Air in the system reduces flow and can damage the pump.
Step three: brush everything
Brush walls, floor, steps, and corners thoroughly. Algae form protective layers that shield them from chlorine; brushing breaks those layers up so sanitizer can reach them. Use a nylon brush on vinyl and fiberglass. A stainless steel brush is typically reserved for stubborn spots on plaster, and should never be used on vinyl. Brush again every day of the treatment.
Step four: lower pH, then shock
Chlorine works best when pH is in the lower part of the normal range, around 7.2 to 7.4. If pH is high, adjust it first according to your acid product's label. Then add chlorine to raise free chlorine to a shock level appropriate for your cyanuric acid. Many pool owners use a chart that ties the target shock level to the stabilizer level; the higher the stabilizer, the higher the free chlorine needed.
Add chlorine in the evening, so the sun does not burn it off immediately. Always read and follow the product label, add chemicals to water and never water to chemicals, and never mix different chemicals together.
Step five: hold the level, do not just dose once
The most important lesson in green pool recovery is that one dose is rarely enough. Retest free chlorine several times a day and add more to hold it at the shock level. When chlorine drops quickly, algae are still consuming it. Keep going. You are done when the water is clear, chlorine holds overnight with only a small loss, and the water passes a basic overnight chlorine loss test.
Step six: clean the filter often
As algae die, they turn gray or white and get caught in the filter, which will clog quickly. Watch your pressure gauge. When it rises notably above its clean baseline, backwash a sand or DE filter or rinse a cartridge filter. During a heavy recovery you may need to do this every day. Vacuum dead algae from the floor, and if there is a large amount of debris, vacuum to waste if your system allows it so the debris does not pass through the filter.
Step seven: rebalance and prevent
Once the water is clear, rebalance pH and total alkalinity, and check calcium hardness if you have a plaster pool. Then build a routine that prevents a repeat: test chlorine and pH a few times a week in hot weather, keep free chlorine in the correct range for your stabilizer level, run the pump long enough each day to turn over the water, brush weekly, and keep baskets clear.
When to call a professional
Call a pro if you see equipment leaks, hear grinding from the pump, smell burning, or find electrical problems. Also consider help if you have repeatedly failed to clear a pool, since there may be a hidden circulation problem. Electricity and water are a dangerous combination; never work on wiring yourself unless you are qualified.
A green pool feels like a disaster, but it is one of the most fixable problems in pool ownership. Patience and steady chlorine win nearly every time.