Why does my pool turn green right after it rains

Manikshaw

Manikshaw

Member
Every time we get a big storm the pool water goes from clear to light green within a day even though my chlorine was fine before the rain hit.
 
Also check your stabilizer heavy rain can dilute the pool and throw the balance off I usually test and adjust CYA right after storms to keep chlorine working.
 
Same thing happened to me it’s all the organics and phosphates washing in from the rain I started adding algaecide before big storms and it stopped turning green.
 
Happens to me almost every rainy spell too, and it can be frustrating. Rain itself is basically untreated water, so when a lot of it falls in the pool it dilutes your chemicals and sometimes brings in dust, pollen, and other organics from the air or runoff. All of that gives algae the perfect window to start growing if chlorine gets weakened.

What I do now is test the water as soon as the storm passes and give it a quick shock if chlorine looks low. Running the pump longer after heavy rain also helps circulate and filter out the extra debris. If you’ve got trees nearby, that runoff can add phosphates, which just feeds the algae more, so a phosphate remover once in a while can make a difference too. Once I started staying on top of those checks after rain, the green water pretty much stopped showing up.
 
Yeah, I’ve noticed the same thing with summer storms. For me, it usually wasn’t just the dilution, but also all the fine stuff washing in that you don’t really see at first, little bits of dirt, leaves, and even fertiliser dust from the yard. That stuff seems to consume chlorine quickly.

What helped a lot was backwashing the filter right after a storm and brushing the walls/floor, because algae spores can settle quickly when circulation slows during heavy rain. If you catch it early, even just upping the chlorine dose before or right after the rain can save you from waking up to a green tint the next day.
 
I’ve dealt with that a few times, and in my case it was mostly the rain knocking the balance out of whack. Even if chlorine tests fine before the storm, the extra water plus all the junk that washes in gives algae a head start. I started shocking right after big rains and letting the pump run longer, and it made a big difference. A quick brush-down after storms also helps keep anything from settling and turning green overnight.
 
I used to get the same “green after storms” problem, and in my case the culprit was metals in the rain runoff. We’ve got old gutters, and whenever heavy rain hit, a little copper would wash in, which made the water tint green almost right away. A metal sequestrant solved it for me, along with shocking and brushing like others have mentioned.


Might be worth checking if it’s algae or metals causing your color shift, the fixes are different, but both tend to show up fast after a storm.
 
I’ve run into this a few times, and for me it was a mix of dilution and junk washing in with the rain. Even when chlorine looked fine the night before, by the next day the pool had that light green tint.

What helped was making a routine out of testing right after storms, running the pump longer, and giving the pool a good brush. If the chlorine reads on the lower side, I’ll throw in a light shock right away. Since I started doing that, the water doesn’t flip green nearly as often.
 
One angle that often gets overlooked is how fast algae can wake up when conditions shift suddenly. A storm doesn’t just add water, it changes temperature, pH, and circulation all at once. If the pool sat still during the rain (pump off, debris covering returns), you basically get a calm window where algae spores can get established before chlorine catches up.

Another sneaky factor is chlorine demand after rain. Even if your test still shows “some” chlorine, it can be busy reacting with everything that blew or washed in, leaving very little actually available to sanitize. That’s why the green tint can appear even when numbers don’t look terrible.

What helped me was treating storms like a mini stress test: skim immediately, brush the pool to break up anything trying to cling, and bring chlorine slightly above normal for a day. Doing that early usually stops the color change before it fully develops.
 
When it turns light green after heavy rain, I usually treat it as an organic spike from rainwater and runoff. I brush the walls and floor right away and check sanitizer because chlorine often gets used up fighting what the storm brought in.
 
I’ll throw in something a little different that caught me off guard the first time it happened.

Sometimes it’s not that the rain “caused” algae, it just exposed that you were already riding the edge. I had my free chlorine sitting at what I thought was fine for my CYA, but I was basically at the bare minimum on the FC/CYA ratio. Water looked perfect. Then we got a hard storm, pool temp dropped about 5 degrees overnight, a bunch of fine dirt blew in, and boom, faint green haze the next day.

What I learned was that rain does three things at once. It dilutes chlorine and CYA a bit, it adds organics, and it can shift pH. If your stabilizer drops even 5 to 10 ppm and you were already dosing to the low end, your effective protection margin shrinks fast. Add extra demand from debris and algae gets a tiny head start.

Now before big storms I bump chlorine slightly above my normal target, not crazy high, just a cushion. After the rain I test CYA and free chlorine, brush everything, and make sure circulation runs longer than usual. I also keep an eye on filter pressure because storms tend to load it up with fine junk, and reduced flow doesn’t help when you’re trying to clear something up.

If you can, check what your CYA and free chlorine are the day after a storm compared to a dry week. A lot of times the pattern shows you were just barely hanging on before the rain hit.
 
You must log in or register to reply here.
Back
Top