Pool Cleanup After a Storm: How to Prevent Algae Before It Takes Over

You don't even need to go outside to know what the pool looks like the morning after a bad storm. Waterline up a few inches. Leaves and pine needles plastered across the surface. And if the rain hung around for more than a day, there's probably a faint green haze starting to build along the deep end walls.

Anyone who's lived through a couple of hurricane seasons from Tampa to Houston has seen this before. What actually matters is what happens in the next 24 to 48 hours. That's the window where you either knock out a quick chemical fix or let things slide — and the difference is roughly the gap between a $20 bag of shock and a week-long green-to-clean job that eats up a few hundred bucks in chemicals and your entire weekend.

Algae spores are in every outdoor pool. They never leave. When your chemistry is right, chlorine keeps them dormant and you never see them. A storm blows that up in about five ways at once.

Rainwater has zero chlorine. When a few thousand gallons dump into a 15,000-gallon residential pool, your free chlorine gets diluted fast — often from 3 ppm down to under 1 ppm in a single afternoon. Below 1 ppm, spores start reproducing. Then the sun comes out the next morning and burns off whatever trace chlorine made it through the rain.

This one confuses people because rainwater is technically acidic — pH around 5.0 to 5.5. So you'd think pool pH would drop. It doesn't. Storm runoff drags in soil, concrete dust, lawn lime, rotting leaves. All alkaline. The net result is pH climbing to 7.8 or 8.0, which makes your remaining chlorine roughly 80% less effective. You can have "enough" chlorine on paper and still wake up to green water because the pH is too high for it to actually work.

The runoff from your yard and your neighbor’s yard will deliver phosphates, nitrates, grass clippings, residues from fertilizers, and tannins from mulch right into your pool. I’ve done tests on pools in the wake of storms and found out that the phosphates rose from 200 ppb to over 1,000 ppb. Once that level is reached, algae starts growing aggressively.

Lost power during the storm? Turned the pump off as a precaution? Either way, your water sat dead for hours or days. No circulation, no chlorine distribution. Algae loves stagnant water.

A 20-minute thunderstorm and a slow-moving tropical depression are completely different events as far as your pool is concerned. The amount of rain dictates how much damage you're dealing with and how aggressive the cleanup needs to be.

Honestly, a quick afternoon storm that drops less than an inch isn't a big deal. Free chlorine might dip half a ppm. pH barely moves. A few leaves blow in. A pool running at 3–4 ppm free chlorine absorbs this kind of rain without any noticeable change in clarity. Algae risk is minimal.

This is where the change starts. 1-2 ppm of free chlorine is lost and there is a good chance that the water levels go beyond the skimmer, thus ending all surface skimming. Pollen, grass and dust enter the pool water from the yard. The chlorine levels were at 2 ppm prior to the rainfall and now the water is at less than 1 ppm with a new food source for the bacteria. The dormant spores have 24-48 hours to grow green.

Three to six inches is where most serious post-storm algae blooms start. You've taken on thousands of gallons of untreated water. Free chlorine is likely zero or close to it. pH has spiked above 7.8 from all the alkaline sediment washing off the deck and lawn. The water looks cloudy, and phosphate readings are probably north of 500 ppb. Left alone, green algae will coat the floor and walls within two days. At this level, a full cleanup is the only option.

Anything over six inches — or any situation where street runoff or yard flooding got into the pool — is a different animal entirely. The water is probably brown or black. You're dealing with heavy silt, motor oil, fertilizer runoff, and possibly sewage. Algae is a given, but it's honestly the least of your problems right now. Water safety and structural integrity come first, and this is not a weekend DIY project.

Figure out roughly how much rain your pool took on, then work through this list in order. Getting the sequence wrong is what turns a two-hour job into a two-week job.

Step 1: Scoop out the big stuff by hand.
Before you touch anything else, grab a leaf rake or deep net and get everything floating on the surface — branches, leaves, palm fronds, whatever the wind brought in. Do it before you turn the pump on. Push all that mess through the skimmer and you'll clog the basket in about ten minutes and burn out the pump motor not long after.

Step 2: Drop the water level if it's too high.
Sitting above the skimmer opening? Get it back down to the middle of the skimmer mouth. A submersible pump works, or set the multiport valve to Waste and let it run for a bit. The skimmer can't do its job when the water's above the throat — it just sits there looking full while all the debris keeps circling.

Step 3: Fix the pH before you touch the chlorine.
This is the single biggest mistake people make after a storm. They see green water, panic, dump in a bag of shock, and wonder why nothing happens. Chlorine barely works above 7.8, and after a storm your pH is almost guaranteed to be up there. Test the water. Add muriatic acid to pull the pH down to 7.4–7.6. Give the pump 20–30 minutes to mix it in, then move on.

Step 4: Hit it with shock.
Cal-hypo or sodium dichlor, either one. Moderate storm damage: aim for 10–15 ppm free chlorine. Heavy damage: push it to 15–20 ppm. Always add shock in the evening. Do it at noon and half of it burns off in the sun before it has a chance to kill anything. Keep the pump running the whole time.

Step 5: Brush every surface — and I mean every surface.
Algae hangs on to walls, steps, ladders, behind the return jets, around the light niche. All the spots you never look at. Brushing breaks up the biofilm and pushes the algae into the water column where the chlorine and filter can actually get to it. Use a stainless steel brush on gunite, nylon on vinyl or fiberglass. Don't rush this step.

Step 6: Let the pump run around the clock.
Keep the filter going 24/7 until the water clears up. Moderate storm, that's usually about a day. Heavy storm, expect 48 to 72 hours of continuous run time. Check the pressure gauge every so often — when it climbs 8–10 psi above where it normally sits after a clean, backwash or pull the cartridge and hose it off.

Step 7: Follow up with algaecide.
Once your free chlorine drops back under 5 ppm, add a dose of polyquat or a copper-based algaecide. This isn't your main line of defense — chlorine did that. Think of it as backup, catching anything that slipped through.

Step 8: Deal with phosphates if the numbers are ugly.
Test phosphates. Under 500 ppb, don't worry about it. Above 500 ppb, use a lanthanum chloride-based remover. Phosphates by themselves won't cause algae, but high levels are the reason some pools stay cloudy or keep going green no matter how much shock you throw at them.