Cloudy water only when pump is OFF?

Manikshaw

Manikshaw

Member
Strangest thing… when my pump runs, the pool looks crystal clear. But the second I shut it off, within 30 minutes the water looks cloudy and hazy. Turn the pump back on clear again. Anyone else seen this?
 
Yeah, that’s usually super fine particles staying suspended only when water’s moving. Could be dead algae or calcium dust. A clarifier or longer filter cycle usually fixes it.
 
Had the same thing happen before. Turns out, it was because the filter wasn’t clean enough, so the debris only got filtered when the pump was running. After cleaning the sand filter, the issue went away! Maybe give that a try.
 
I’ve noticed this too in my pool a while back. For me it was just super fine stuff that would settle once the water stopped moving, so it looked cloudy when the pump was off. A floc treatment followed by a good vacuum to waste cleared it up. Since then I also keep the filter running a little longer each day and it hasn’t come back.
 
That usually happens when really fine particles stay suspended while the water’s moving. When the pump stops, they spread out and make the pool look cloudy. Check your filter, it might need a deep clean. Using a clarifier or occasionally running the pump a bit longer can help keep the water clear even when it’s off.
 
Yep, seen this and it’s one of those things that feels backwards. When it happened to me, the water looked perfect as long as there was movement. Shut the pump off and within half an hour it went hazy, like someone poured milk in it. Nothing changed chemically in that time, so it wasn’t sanitizer dropping or anything like that.

What’s really happening is you’ve got ultra fine stuff in the water, dead algae, calcium dust, pollen, just tiny particles that your filter isn’t quite catching. When the pump is on, everything stays evenly suspended and the light scatters less, so it looks clear. Once the water goes still, those fines spread out and settle unevenly, which makes the water look cloudy even though it’s the same water. My filter pressure looked normal too, which threw me off, but it just wasn’t polishing the water.

What fixed it for me was switching focus from killing to removing. Deep cleaned the filter, slowed the flow a bit so it could actually grab the fines, and used a light dose of aquadoc clarifier to help those particles clump instead of floating forever. After a day or two, the water stayed clear even with the pump off. If the cloudiness only shows up when circulation stops, that’s almost always a filtration problem, not a chemistry one. Anyone else notice this after shocking or clearing algae when everything’s technically already “dead”?
 
i’ve seen this and it’s almost always fine particles that only stay suspended when the pump is running, once flow stops they settle and make the water look hazy, what fixed it for me was brushing to keep stuff moving, checking filter pressure, and using a small dose of aquadoc water clarifier from mavaquadoc so the fines could clump and actually get caught instead of just floating around.
 
Yep, that’s a classic sign of ultra-fine particles in the water and a filter that isn’t quite polishing them out.

What’s throwing you off is that circulation makes it look better. When the pump is running, everything stays evenly suspended and the light passes through more cleanly. As soon as the water goes still, those microscopic particles (dead algae, pollen, calcium dust, very fine dirt) spread out and settle unevenly, which makes the pool look hazy even though nothing “new” happened.

A few key points that usually line up with this:
  • Chemistry is often fine.
    If it clouds up in 30 minutes, it’s not chlorine suddenly dropping — that’s way too fast.
  • Filter is working… just not enough.
    Pressure can look normal, but the filter may be slightly loaded, channeling (sand), or moving water too fast to catch fines.
  • Common after shocking or clearing algae.
    Everything’s dead, but not removed yet.
What usually fixes it:
  • Give the filter a proper deep clean (sand, cartridge, or grids).
  • If you can, slow the flow a bit so the filter can grab smaller particles.
  • Run the pump continuously for a day or two to finish the cleanup.
  • If the water is otherwise balanced, a small dose of clarifier can help clump fines so the filter can actually catch them (don’t overdo it).
If the pool only looks cloudy when the pump is OFF, that’s almost always a filtration issue, not a chemistry one. Once the fines are truly removed, the water will stay clear whether the pump is running or not.
 
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