Pool Care After Rain: How to Stop Cloudy, Green Water
It doesn't take a storm to throw your water out. Here's what even a passing shower does to your pool, and how to handle it before, during and after.
Most pool owners brace for the big storms and shrug off a light shower. The problem is that rain doesn't need volume to cause trouble. Even a short drizzle washes dust, pollen and organic matter out of the air and off your surrounds, nudges your pH down and hands algae exactly what it needs. Add a warm, humid afternoon afterwards and you've got green water waiting to happen.
Typical rainwater
pH 5.6
naturally acidic
Ideal pool pH
7.2 to 7.6
Run the filter after rain
24 hours
continuously
What rain actually does to your water
The water itself is only part of the story. It's what the rain brings with it that does the damage.
Drags your pH and alkalinity down
Rain picks up carbon dioxide on the way down, which makes it mildly acidic. A light shower won't crash your chemistry on its own, but it chips away at your total alkalinity and pH. Low pH burns through your chlorine faster, makes the water corrosive to equipment and surfaces, and leaves swimmers with stinging eyes.
Uses up your chlorine
Everything rain washes in, from dust and pollen to leaves and garden runoff, has to be broken down by your sanitiser. Your chlorine gets used up fighting that load, which leaves less available to keep bacteria and algae in check. Heavier rain also dilutes your salt and stabiliser levels.
Feeds algae
Rain carries phosphates and nitrates from soil, fertilised gardens and the air itself. Algae spores ride in with it too. Combine that food source with lower chlorine and warm weather, and an algae bloom can take hold within a day or two.
Clogs your skimmer and filter
Leaves, grass clippings and dirt end up in the skimmer basket and the filter. A full basket restricts flow, and a loaded filter lets fine particles circulate, which is where cloudy water after rain usually comes from.
How to handle the rain
A bit of preparation means a quick tidy up afterwards rather than a week of chasing green water. Jump to the stage you need.
Before the rain
If showers are forecast, give your pool a head start while the weather's still dry.
Test and balance your water
Run a quick check with test strips or a test kit and get your total alkalinity and pH sitting in range. Balanced water handles the acidic hit from rain far better than water that's already drifting low.
Lift your chlorine and add algaecide
Bring your chlorine up to the higher end of its normal range so there's some in reserve. On a salt pool, that means turning up your chlorinator output or extending its run time for a day or two.
A preventative dose of algaecide gives you an extra layer of protection against whatever washes in.
Empty the baskets and clean the filter
Start with an empty skimmer basket and pump basket, and a clean filter. Hose down a cartridge filter, or backwash and rinse a sand or media filter if the pressure has crept up. That gives your system full capacity for the debris load when it arrives.
Tidy the surrounds and cover up
Sweep paths and pavers, trim back overhanging branches and move pot plants or loose mulch away from the edge. If you have a pool cover, put it on. It's the single easiest way to keep rain and debris out, and our guide on caring for your pool blanket will help it last.
While it's raining
Mostly this is about leaving things alone and letting the system do its work.
Keep the pump running
Circulation stops contaminants settling and keeps your sanitiser moving through the water. In light rain, there's no reason to switch it off.
The exception is a thunderstorm. If there's lightning around, or water is pooling around your equipment, turn the power off at the switch until it passes.
Hold off on chemicals
Anything you add during rain gets diluted, or knocked off balance by what's coming in. Save your chemicals for once the weather clears and you can test properly.
Stay out of the water
Rain itself isn't dangerous, but it often comes with lightning, and runoff means your sanitiser is under pressure. Give it a break until you've retested.
Once it clears
Work through these in order. Physical clean up first, then chemistry, then let the filter catch up.
Clear the debris
Skim the surface, empty the skimmer and pump baskets, then brush and vacuum the walls and floor. Getting the organic matter out first means your chlorine isn't wasted on it.
Check the water level
If the level is above the middle of the skimmer opening, the skimmer can't pull debris off the surface properly. On a sand or media filter, use the waste setting on the multiport valve to lower it. Otherwise use a submersible pump.
Test, then rebalance
Test your water and correct total alkalinity first, then pH. Alkalinity acts as a buffer for pH, so fixing it first saves you chasing your pH up and down.
After heavier rain, check your salt and stabiliser levels too, as both get diluted.
Shock the pool
Once the water is balanced, shock dose to knock out anything the rain brought in. Do it in the evening so the sun doesn't burn the chlorine off before it does its job. If algae keeps coming back after rain, a phosphate remover takes away its food source.
Run the filter and check the pressure
Run the filter continuously for 24 hours to clear fine particles. If the gauge climbs 8 to 10 PSI (about 55 to 70 kPa) above your clean reading, clean a cartridge filter, or backwash and rinse a sand or media filter.
If a cartridge filter's pressure stays high after a thorough clean, it's time for a replacement filter cartridge.
Still cloudy? Clarifier or flocculant
If the water is balanced, chlorinated and the filter has had a full day but it's still cloudy, the particles are too fine for the filter to catch. For a light haze, start with a liquid clarifier, which clumps fine particles just enough for the filter to grab them.
If it's properly milky, go straight to a liquid flocculant. It binds the particles into heavier clumps that sink to the floor. Turn the pump off, leave it to settle overnight, then vacuum the sediment to waste rather than through the filter. On a sand or media filter, use the waste setting. On a cartridge filter, use a submersible pump or a vacuum that discharges outside the system.
Rain and your pool: common questions
Should I run my pool pump in the rain?
Yes, in light rain keep it running so the water keeps circulating and your sanitiser keeps working. Turn it off at the switch if there's lightning around or water is pooling around your equipment.
Should I shock my pool after rain?
Usually, yes. Once you've cleared the debris and rebalanced alkalinity and pH, a shock dose in the evening breaks down whatever the rain washed in and stops algae getting a foothold.
How long after rain can I swim?
Once you've tested and your chlorine and pH are back in range. If you've shocked the pool, wait until chlorine drops back to normal swimming levels, following the directions on the product label.
Does rain lower chlorine?
Yes. Heavy rain dilutes it, and even light rain brings in dust, pollen and organic matter that use it up. Rain also lowers pH, which makes chlorine burn off faster.
Can rain turn my pool green?
It can, often within a day or two. Rain carries in phosphates, nitrates and algae spores while your chlorine is busy elsewhere, and warm weather does the rest. Lifting chlorine and adding algaecide before the rain is the best prevention.
A light shower is a quick fix if you're ready for it, and a week of green water if you're not. Get rain ready before the next one. Start with test kits, algaecides, shock treatments and clarifiers, with fast delivery across Australia.
Not sure what your water needs? Call us on 1800 648 363 and we'll help you get it back on track.
