It is a common surprise: the pool looked fine a few days ago, and now the water is green. Algae can take over surprisingly fast, especially in the Sacramento summer. The good news is that the cause is almost always one of a few simple things.
Algae Needs Only a Small Opening
Algae spores enter every pool all the time, carried by wind, rain, leaves and swimmers. Chlorine kills them before they can grow. The moment free chlorine drops close to zero, the spores start multiplying, and in warm water the population can double in hours. That is why a pool can seem to turn green overnight.
The Most Common Causes
Chlorine ran out
Hot weather, strong sun, heavy swimming, a pool party or a rainstorm can use up chlorine much faster than usual. If no one tests and adds chlorine for several days, there is nothing left to stop algae.
Low or missing stabilizer
Outdoor pools need cyanuric acid (stabilizer) to protect chlorine from sunlight. Without it, the sun can burn off most of the chlorine in a few hours.
High pH
When pH climbs above about 7.8, chlorine becomes much less effective. The test may show chlorine, but it is not doing much work.
The pump stopped or ran too little
A tripped breaker, a failed timer or a pump that stopped priming means the water is not filtered or mixed. Still water, especially in corners and on steps, is where algae starts.
A dirty or failing filter
A clogged filter cannot remove dead algae and debris. A torn cartridge or broken laterals let dirt pass straight back into the pool.
Phosphates and debris
Leaves, lawn fertilizer and dirt add nutrients that feed algae, making it harder to keep chlorine ahead.
What to Do First
- Check that the pump is running and water is coming out of the return jets.
- Empty the skimmer and pump baskets.
- Test free chlorine and pH. Lower pH if it is high so chlorine works better.
- Brush the walls and floor to break up the algae.
- Shock the pool with enough chlorine for the amount of algae and run the filter continuously.
- Clean the filter often while the algae clears.
Light green water can often be cleared in a day or two. Dark green or swampy water usually needs much more chlorine, repeated brushing and several filter cleanings.
How to Keep It From Happening Again
Keep free chlorine in range at all times, keep stabilizer at the right level, run the pump long enough each day and test at least once a week. In summer, test more often after heat waves and parties. If you are away often, a weekly pool service is the simplest way to keep the water from turning.
Frequently Asked Questions
Can a pool turn green overnight?
Yes. If chlorine runs out in warm water, algae can grow fast enough that the color change seems to happen overnight.
Does shocking the pool always fix green water?
Shock is the main step, but it works best with brushing, a running pump, a clean filter and pH in range. Very green pools may need several rounds.
Should I drain a green pool?
Usually not. Most green pools can be cleared with chlorine, brushing and filtration. Draining is only needed in rare cases.
Need a Hand With Your Pool?
H2O Pool Care is a licensed, family-owned pool service serving Sacramento and Placer counties since 2017. If your pool is already green, our green pool treatment usually brings it back to clear water in 2-3 days. Call (916) 335-3559 or request a free quote.
