Why Your Pool Turns Green After a Storm (and What to Do)

Why does a pool turn green after a storm?

Because a storm weakens your chlorine and feeds the algae at the same time. Rain dilutes the sanitizer. Wind and runoff add leaves, dirt, and nutrients. Algae spores are always in the air, so once chlorine is too low, they have food and room to grow. This pattern is called post-storm algae.

Hydra Pool Services, a North Texas company, says rain introduces phosphates, nitrogen, and organic debris while diluting chlorine. It says post-storm blooms are common in North DFW from March through October. It adds that in warm water a bloom can start in 24 to 48 hours if chlorine drops to zero. This is a company blog post, not a lab study, so treat the timing as a rough guide.

What happens in the water?

Each part of the storm pushes in the same direction:

Stagnant water makes it worse. Hydra notes that poor circulation creates dead zones where algae sets in first, then spreads. If the pump was off for a day or two, that is a likely factor.

How do I know it is algae?

Look and test. Light green or teal water you can see through is an early bloom. Dark green water that hides the bottom is a heavier one. Our post on green, black, and mustard algae shows how to tell types apart, since each needs a different plan.

A test is more reliable than a look. CDC recommends a DPD test kit for the most accurate chlorine and pH readings, and says test strips are less accurate. It recommends pH 7.0 to 7.8 and free chlorine of at least 1 ppm in pools. If you use cyanuric acid, which is a stabilizer, CDC recommends at least 2 ppm. Hydra adds that high cyanuric acid makes chlorine less effective, a sneaky cause of green water after storms.

What should I do first?

Take these steps in order, and follow the label on every product.

1. Stay out of the water until your readings are in range.
2. Test chlorine, pH, and stabilizer before adding anything.
3. Remove debris. Use a leaf net, then empty the skimmer and pump baskets. Hydra says to hold off vacuuming at first, since it clogs the filter.
4. Check the filter. Clean or backwash it if pressure is high. Dead algae clogs filters fast.
5. Brush the surfaces. Brushing breaks algae loose so chlorine can reach it.
6. Raise chlorine with a shock product only as the label directs.
7. Run the pump to move water through the filter.

Pool Founder, a pool service software company, advises lowering pH to about 7.2 before shocking, because chlorine works better at lower pH. Hydra says the same. Check your pH first, and adjust only as your product label allows.

For step-by-step recovery, see our guide on how to clear a green pool, or read green pool DIY steps and when to call a pro.

How soon will the water turn green?

It depends on how much chlorine is left and how warm the water is. Hydra says a heavy rain, a week away from the pool, or a pump that ran too little can each be enough in a Texas summer. It lists water above 80 degrees F from May through September and strong UV that burns through chlorine in hours. Fall storms can still cause trouble while the water stays warm. The safest habit is to test the water within a day of a big storm, even if it looks fine. A clear pool with no chlorine left is one warm day away from a bloom.

How do I handle the chemicals safely?

CDC says pool chemicals can injure people when mixed together or handled without protection. Its guidance includes storing each chemical separately, keeping them in original labeled containers, keeping them dry, and keeping them away from heat and flames. Never mix products. If a label and a blog post disagree, follow the label.

How can I lower the risk before the next storm?

• Keep free chlorine in range, and test again after a storm.
• Keep the pump running on a normal schedule, including after a power loss.
• Empty baskets and brush walls and steps on schedule.
• Check the filter and stabilizer level a few times a year.
• Do not wait if the water starts to tint. Early action costs less effort than a full recovery.
• Ask a pro to check the filter and pump if the water clouds again soon after cleanup.

Weekly service covers many of these. Marina’s weekly service includes water balance, emptying baskets, backwashing filters as needed, and vacuuming and brushing as needed.

Next step

If your pool is already green, read about algae treatment from Marina Pool Services. Call (817) 896-1120 or use the quote form to request a visit.