Rain falling on a pool is not just water. Rainwater is slightly acidic, typically sitting at pH 5.6 to 6.0, and it brings with it organic matter, phosphates and other compounds washed from the air and surrounding garden. A heavy rain event can shift pool chemistry noticeably within hours - sometimes enough to trigger cloudiness or the start of an algae bloom if left uncorrected.
The good news is that rain-related problems are among the easiest to prevent and the most straightforward to correct once you know what to check. This guide walks through exactly what rain does to pool water, what to test and correct afterwards, and how to minimise the impact of future rain events.
What rain does to pool water
Understanding the specific effects of rain helps you target the right corrections rather than treating everything at once.
Rain lowers pH
Rainwater is naturally acidic due to dissolved carbon dioxide and other atmospheric compounds. When acidic rain enters the pool, it pushes the pH downward. Even a pool with pH sitting at 7.4 can drop to 7.0 or below after sustained heavy rain. At low pH, chlorine becomes overly aggressive, corrosion risk increases, and swimmer comfort decreases.
Rain dilutes chlorine
Rain does not contain chlorine. Every litre of rain that enters the pool dilutes the free chlorine concentration proportionally. After a storm that adds a significant volume of water - particularly in a small above-ground pool where the surface area is large relative to total volume - free chlorine can drop to levels that leave the water under-protected.
Rain brings phosphates and organic matter
Rainwater picks up phosphates, nitrogen compounds, dust, pollen and organic debris from the atmosphere and from the surrounding garden as it runs into the pool. Phosphates are the primary nutrient source for algae. An influx of phosphates after rain, combined with temporarily low chlorine, creates ideal conditions for algae growth within 24 to 48 hours.
Rain washes in debris
Leaves, grass, soil and insects can wash into the pool during and after rain. These all add to the organic load in the water, consuming chlorine as they break down. Clear debris from the pool surface with a net as soon as possible after a rain event.
Heavy rain can overflow the pool
If the water level rises above the skimmer opening, the skimmer cannot function and water circulation is compromised. An overflowing pool also loses some of its treated water, which is replaced by rainwater.
What to check after rain: a checklist
Run through these checks in order after any significant rain event.
Test pH first. This is the most likely parameter to have shifted. Target 7.2 to 7.6. If pH has dropped below 7.2, correct it before adding chlorine - the effectiveness of any chlorine you add depends on the pH being in range.
Test free chlorine. After heavy rain, free chlorine may have dropped below 1 mg/l. If it is below 1 mg/l, add chlorine to bring it back into the 1 to 3 mg/l range. If it has dropped very low (below 0.5 mg/l), consider a light shock treatment rather than just a maintenance dose.
Check the water level. The water surface should sit at approximately mid-skimmer level. If the pool has overflowed or the level is too high, lower it using the backwash or waste setting before adding chemicals. Adding chemicals to an overfull pool is wasteful - any correction you add will partly drain away.
Check water clarity visually. Clear water after rain is a good sign. Hazy or slightly greenish water within 24 hours of rain suggests the chemical correction needs to happen quickly. For cloudy water, see our guide on clear pool water .
Clear debris. Use a net to remove leaves, insects and any other debris from the water surface and floor before they break down and consume chlorine.
Step-by-step correction after heavy rain
Follow this sequence for correcting pool water after a significant rain event.
Step 1: Lower the water level if needed
If the pool has overflowed or is above the midpoint of the skimmer mouth, set the multiport valve to waste and run the pump until the level drops to mid-skimmer. Do this before adding any chemicals.
Step 2: Remove debris
Net out all visible debris. Brush the walls and floor if organic material has settled on the surfaces. Run the pump to circulate the water and allow the filter to capture any suspended particles.
Step 3: Test the water
Test at minimum: pH, free chlorine and ideally total alkalinity. If you have a test kit that covers phosphates, this is a good time to test those too, especially if you have had algae problems in the past.
Step 4: Correct pH
If pH has dropped below 7.2, add pH plus (sodium carbonate). Pre-dissolve granular pH plus in a bucket of pool water before adding. Run the pump and retest after 2 to 4 hours. Do not over-correct - add smaller amounts and retest rather than adding a large dose and overshooting.
If pH is correct or only slightly low, you can move to chlorine correction simultaneously.
Step 5: Top up chlorine
Add chlorine granules or tablets to bring free chlorine back to 1 to 3 mg/l. For a very light rain event with only a small drop in chlorine, topping up with a standard maintenance dose is sufficient.
After heavy or prolonged rain - especially if organic material entered the pool - consider a light shock treatment: raising free chlorine to 5 to 8 mg/l. This oxidises the additional organic load and prevents it from feeding algae.
Tip
In warm weather (above 20 degrees Celsius), a shock treatment after heavy rain is good preventive practice even if the water looks clear. The combination of warm temperature, diluted chlorine and an influx of phosphates makes algae growth very rapid. A proactive shock is far easier than treating an algae bloom 48 hours later.
Step 6: Run the filter
After adding corrections, run the pump for at least 6 to 8 hours to circulate the treated water and allow the filter to remove any suspended particles. Check the filter pressure and backwash if needed - rain events often bring in fine particles that can clog the filter faster than usual.
Preventing rain damage
The most effective prevention strategy is a pool cover. A cover that is on before rain falls protects the water from dilution, keeps out debris and organic matter, and dramatically reduces the pH drop caused by acidic rain. Even a partial cover over part of the pool helps.
Tip
A solar cover does not seal the pool completely - some rainwater can enter at the edges. However, a solar cover still dramatically reduces the dilution effect and keeps out most debris. The cover also retains heat, so a pool with a solar cover is more stable in every respect during a rain event.
Other practical preventive measures:
- Keep chlorine on the higher end of the normal range (2 to 3 mg/l rather than 1 mg/l) during periods of forecast heavy rain
- Maintain pH in the middle of the target range (7.3 to 7.5) so a pH drop does not immediately put you out of range
- Keep total alkalinity at 100 to 120 mg/l - higher alkalinity acts as a buffer and slows pH change from acidic rain
What if the pool overflows?
An overflowing pool has lost some of its treated water and gained a corresponding volume of rainwater. The priority is:
- Lower the water level to mid-skimmer before adding any chemicals
- Test and correct pH - this is usually the most affected parameter
- Test and correct chlorine
- Run the filter for an extended period to clear any turbidity
If a large proportion of the pool volume was displaced by rainwater (rare but possible in very small pools during extreme storms), the water chemistry is essentially reset and you may need a more thorough re-balancing including checking total alkalinity and calcium hardness.
The water level after correcting should sit at the midpoint of the skimmer mouth. This position allows the skimmer to draw surface water efficiently - which is where most debris and organic material accumulate.
Product recommendations
pH Minus 5kg - Sodium Bisulphate
✓ Our pick: Fast-dissolving granular pH reducer. Essential after rain events that push pH below 7.2. Pre-dissolve in a bucket before adding to pool.
Pool Shock Granules 5kg
✓ Our pick: Fast-dissolving calcium hypochlorite shock. Ideal for preventive shock treatment after heavy rain events in warm weather.