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Skim&Soak

Green Water & Algae

Green Pool After Rain

A storm does three things to a pool at once, and the combination is why so many pools go green within two days of one.

By Scooter M.Last reviewed September 2, 2026How we work the numbers out
Rain falling onto the surface of a swimming pool during a storm

What the rain actually did

  1. **It diluted your chemistry.** Two inches of rain on a 20 by 40 pool is roughly 1,000 gallons of unchlorinated, unstabilized water. On a 20,000-gallon pool that is a 5% dilution of everything at once - free chlorine, cyanuric acid, alkalinity.
  2. **It carried things in.** Runoff from decking, lawns and flowerbeds brings organic matter, dirt and fertilizer. Fertilizer is phosphate, and phosphate is algae food.
  3. **It shifted pH.** Rain is mildly acidic, and a large enough volume moves pH and alkalinity down. Heavy, prolonged rain does this measurably.
  4. **It added chlorine demand while removing chlorine.** Everything that washed in consumes sanitizer, at the same moment there is less of it.

Any one of those alone is manageable. Together they are why the 48 hours after a storm is the highest-risk window in a pool season.

The 24-hour checklist

  • **Skim and empty the baskets first.** Whatever blew in is consuming chlorine while it sits there, and the pump basket is where it ends up.
  • **Test free chlorine.** Expect it to be low. Bring it back to your normal target before doing anything else - see the dose tables.
  • **Test pH and alkalinity.** Both usually drop. Correct alkalinity before pH, because alkalinity is what stops pH bouncing - how to balance pool water.
  • **Check cyanuric acid if the rain was heavy.** Significant dilution takes stabilizer with it, and an unstabilized pool loses chlorine to sunlight fast.
  • **Run the filter continuously for at least a day.** It is dealing with a debris load it did not have on Friday.
  • **Brush.** Whatever settled on the walls is the foothold a bloom needs.

Why phosphate is the part people miss

Chlorine and pH you can see on a test strip within a minute, and most people check them. Phosphate is invisible on a standard kit, arrives with runoff, and does not go away on its own - chlorine does not remove it.

HTH's own phosphate remover page states that levels above 200 to 500 ppb make algae harder to control. A pool that keeps re-blooming after every storm, despite chlorine that looks fine, is usually being fed. See are phosphates bad for your pool.

Prevention, in order of how much it helps

  • **Keep cyanuric acid in range going into storm season.** A stabilized pool holds its chlorine through the dilution far better than an unstabilized one.
  • **Direct runoff away from the pool.** Decking that drains toward the water is delivering fertilizer every time it rains, and this is a landscaping fix rather than a chemical one.
  • **Do not fertilize near the pool before rain is forecast.** Obvious once stated, rarely thought about.
  • **Run the pump through the storm if it is safe to do so.** Circulation is what keeps sanitizer distributed while everything is arriving at once.
  • **Dose a polyquat algaecide before a forecast storm**, not after. It is preventative chemistry and it works best before the event.

If it is already green

Then this page is history and you want the fix: how to clear a green pool fast. Come back to the prevention list once the water is clear, because a pool that went green after one storm will go green after the next one for the same reasons.

Questions people actually ask

Why does my pool turn green after it rains?
Rain dilutes your chlorine and stabilizer, washes phosphate and organic debris in from the surrounding ground, and shifts pH down - all at the same time. The pool has less sanitizer and more demand simultaneously, which is exactly the condition a bloom needs.
Should I shock my pool after heavy rain?
Test first. If free chlorine has dropped below your normal target, bring it back up; if the water is still clear and chlorine is holding, a full shock is unnecessary. Shocking every storm as a reflex adds calcium or stabilizer you may not need.
Does rain lower pool pH?
Yes. Rain is mildly acidic, and heavy or prolonged rain moves both pH and total alkalinity down. Correct alkalinity first, because it is the buffer that keeps pH from bouncing back out of range.
How long after rain does a pool go green?
Typically 36 to 72 hours if nothing is corrected - long enough that the connection to the storm is not obvious by the time it happens. Testing within the first 24 hours is what breaks that sequence.

Sources

Every figure on this page traces to one of these. Where a number could not be sourced, the page says so rather than estimating.

Why there is nothing to buy here: This is a timing page rather than a shopping page. If the pool has already gone green the products are on [best pool shock](/green-water/best-pool-shock); if it has not, the useful action is a test, which costs nothing you do not already own.

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