Pool Maintenance After Heavy Rain

Why Heavy Rain Affects Your Pool

A heavy downpour does more than fill your pool with extra water. Rainwater is naturally acidic and dilutes your pool's chemical balance. Runoff from surrounding surfaces carries dirt, fertilisers, lawn clippings, bird droppings, and other organic matter into the water. Together, these can drop your pH, reduce your sanitiser levels, and create conditions where algae and bacteria can take hold quickly.

Auckland's climate means heavy rain events are common year-round, not just in winter. Knowing what to check after a storm means you can act before small problems become bigger ones.

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What Heavy Rain Does to Your Pool Chemistry

It lowers pH

Rainwater is typically acidic. When a large volume enters your pool, it can pull the pH down below the safe range. Low pH makes the water corrosive, uncomfortable for swimmers and potentially damaging to pool surfaces and equipment over time.

It dilutes chlorine

The influx of fresh water reduces your chlorine concentration. At the same time, the organic material carried in by runoff increases chlorine demand. The result: your sanitiser gets used up faster than normal, leaving the water less protected.

It introduces phosphates and organic matter

Fertilisers, leaf litter, and soil runoff are rich in phosphates and organic compounds. These act as a food source for algae. If your chlorine is already low after rain, algae can begin to establish within a day or two.

It can raise or lower total alkalinity

Depending on what enters the pool, total alkalinity can shift in either direction. Low alkalinity makes pH unstable and harder to correct. High alkalinity can cause cloudiness and scale.

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What to Check After Heavy Rain

Work through these checks in order. Don't skip ahead, each one informs the next.

1. Check the water level

Heavy rain often overfills pools. An overfull pool reduces the effectiveness of your skimmer, which relies on surface tension to pull in debris. If the water level is above the midpoint of the skimmer opening, drain some water down before doing anything else.

2. Remove debris

Use a leaf net or skimmer to clear leaves, twigs, and other debris from the surface and floor. The sooner you remove organic matter, the less it breaks down and depletes your chlorine.

3. Test the water

Test your pool water as soon as possible after a rain event. The key parameters to check are:

• pH, aim for the correct range for your pool type
• Total alkalinity, this buffers pH and should be checked before adjusting pH
• Free chlorine, confirm your sanitiser level is adequate
• Phosphates, elevated levels after runoff can fuel algae growth

If you don't have a reliable test kit at home, Sparkling Pools offers on-site water testing across Auckland. A technician comes to your home and tests everything on site.

4. Check your filter and skimmer baskets

After heavy rain, skimmer and pump baskets fill up quickly. A blocked basket restricts water flow and puts strain on your pump. Empty them and check that your filter is running properly.

5. Run your filter longer than usual

After a significant rain event, run your filtration system for an extended period to help clear the water and process any fine particles that have entered the pool. This is especially important if the water looks cloudy or discoloured.

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How to Rebalance Your Pool After Rain

Once you have your test results, work through corrections in this order. Adjusting in the wrong sequence wastes chemicals and can make the problem worse.

1. Adjust total alkalinity first. Alkalinity stabilises pH, so correct it before touching pH. If it's low, add a suitable alkalinity increaser. If it's high, an acid dose will bring it down, but go carefully and retest.
2. Adjust pH. Once alkalinity is in range, bring pH into the correct zone. Use pH increaser (sodium carbonate) if it's low, or pH reducer (sodium bisulphate or muriatic acid) if it's high.
3. Shock the pool. After rain, a shock dose of chlorine helps oxidise the organic material that's entered the water and restores your sanitiser level. This is especially important if the pool looks cloudy or if you've had a lot of runoff. Shock in the evening to avoid UV degradation.
4. Treat for phosphates if needed. If your test shows elevated phosphates, a phosphate remover will reduce the food source available to algae.
5. Check again after 24 hours. Retest the water the following day to confirm levels have stabilised before swimming.

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What People Commonly Miss

Waiting too long to act

The longer you leave an unbalanced pool after rain, the harder it becomes to correct. Algae can begin to establish within 24-48 hours if chlorine is low and conditions are right. Test and treat promptly.

Forgetting the filter

Many pool owners focus on chemistry and overlook the filter. A dirty or overloaded filter won't clear the water effectively, even if your chemicals are correct. Clean or backwash your filter after a heavy rain event.

Assuming the pool is fine because it looks clear

Water can look clear but still be chemically out of balance. Low pH and low chlorine aren't always visible. Always test, don't rely on appearance alone.

Not checking the pool surrounds

After rain, check the area around your pool for debris, soil displacement, or anything that could continue washing into the water. Clearing the surrounds reduces ongoing contamination.

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When to Call a Professional

Most post-rain maintenance is straightforward if you have a test kit and the right chemicals on hand. However, call a pool professional if:

• The water has turned green or is heavily clouded and isn't responding to treatment
• You're unsure how to interpret your test results or which chemicals to add
• Your equipment, pump, filter, or chlorinator, isn't operating normally after the rain event
• You've had significant flooding or debris ingress that may have affected your equipment

Sparkling Pools provides on-site water testing and pool servicing across Auckland. If your pool isn't coming right after rain, a technician can assess the water and equipment on site and advise on the right course of action.