Swimming Pool Blog

Green to Cloudy Blue to Clear

cloudy pool water
Rob Cox August 25, 2015


My friend and neighbor has had a little problem this summer with the pool water. Specifically the color and clarity of the pool water. Like a diamond, a swimming pool has to have the right color and clarity - and you know when things aren't quite right.

It often starts with a lack of filtration and/or sanitation, and our friend was admittedly a bit negligent in both areas. It happens to every pool owner from time to time, if your chlorine gets too low, or you aren't running the filter pump long enough during hot sunny periods.

It starts off slowly, a slight haze in the morning, which obscures the deep end by nightfall. The next morning the algae begins to bloom in the sunlight and by nightfall of the second day you've got a full blown green pool.

It doesn't take more than 2-3 days to go from blue to green. But - it can take 2-3 weeks to bring it back from green to blue. Here's a picture story of the long road back, and some support I gave along the way.


Q: After repairing the pump I was able to clean the water with shock and clarifier, but it gets dirty very fast.  It was never fully clean, but at least I could see the bottom of the pool, then removed all the bottom dirt with the manual vacuum through the skimmer.  In total I put clarifier 3 times, about 6 oz each time. After the big storm 3 days ago, the pool was green again.  I put 2 pounds of shock yesterday evening but this morning it was still very green and I could not see the bottom (see pictures).  Should I simply put more shock and more clarifier?!? You said previously that the clarifier was a coagulant, is there any health risk associated with putting too much of it in the pool, what happens to the clarifier in the pool over time, does it just disappear like chlorine?

A: During these hot weeks, I would suggest that you run the filter during the hottest part of the day like 10-4pm, to keep it filtered and chlorinated. Running only at night can create some problems for your water...For the current situation, run the pump 24/7, but don't use more clarifier, just use shock, 2-3 lbs (with a low pH around 7.2), add it until it turns a blue/grey color. Brush the pool well, and run the cleaner for a few hours each day, maybe in 2 or 3 short shifts, to help circulation. No health risk with the clarifier, but using too much can cause the clarifier to act as a dispersant, or the opposite effect of what you want. Just follow the label guidelines, and resist the urge to retreat within a week, after a week you could re-dose the pool. What happens to the clarifier polymers is that they grow larger with particulate matter and get filtered out of the water, trapped by the filter. Then they are discharged during backwashing. Note: The directions for PhosFree say to kill all algae before treatment - oops, not done.

Q: Rob, here are 2 pictures I took this morning, the pump has now been running for about 48 hr and the water is still green!,  I added a bit of phosfree yesterday, so altogether I must have put about 2 liters. Hopefully I will receive the new batch of shock soon, should I add 10 lb at once or go progressively?

A: Such trouble! I can't believe it's not better yet - are you sure that you can trust your pH test? Old reagents or strips can give faulty results. If pH is 7.8 or higher, algae is hard to kill. The whitish plume when brushing is likely shock dust. If you have a manual vacuum set-up, you can vacuum it to waste (set multiport valve to waste). Keep the chlorine level high - 2-3 ppm, with tablets , and if it's still greenish when you get the shock, yes, I think 10 lbs may be in order, to break whatever is causing this. When you backwash, let it run for about 3 minutes, to get a good backwash, and then use the Rinse setting for about 15 seconds, before returning the valve to the Filter position.

Q: Here are 4 pictures taken this morning, taken after the pump ran all night the water is a bit cleaner but still foggy and I can not see the bottom the pump was at 20 PSI when I put the shock yesterday late afternoon, it was at 21 PSI this morning, I did a short Back-wash and the pump now shows 19 PSI.  The pump ran all night and is now running with only an hour stopped 8-9 am. Because there was so much shock in the pool, I turned off the chlorinator last night, should I reopen it? When should I add the clarifier? Now? Or let the shock work a little be more and put the clarifier in late afternoon?

A:  Ahhh, that's better, a nice shade of blue - I think you got it back! Add the clarifier, according to directions for treatment and dosage, and resist the urge to backwash until the pressure rises to 25 lbs, as sand filters will filter more effectively when they are somewhat dirty.  If you are concerned about the high pressure, you can open the (RayVac) cleaner line a little bit, to reduce the tank pressure by a few psi...You can keep the chlorinator off for now, but check your chlorine carefully every day (pH and Alkalinity too), so it never drops below 1 ppm. When the chlorine level drops back down to 2-3 ppm, turn the chlorinator back on. Keep running the filter 23 hours per day until it fully clears up, then cut it back to 15 hours per day - maybe 5am-12pm and 5pm-12am? That's 14 hours...it's still hot! 

Q: Here are the results of 12 oz of Aluminum Sulfate and about 15 hrs of filtering. There is a net improvement and we can see some of the debris at the bottom of the pool, but we can not see the deep end of the pool yet. Should I simply keep filtering or add more magic powder? If I need to add something, I have both the concentrated clarifier and the aluminum sulfate; which one would me most effective at this time?

A:I would try another dose of aluminum sulfate, in the same manner as before, backwash and rinse the filter again, then add another 12 oz of alum - if you haven't added clarifier in a few days, add a dosage at the same time. Also check chlorine if you haven't - to be sure it' s 1.0 or higher, and Alum works best with a higher pH, so recheck/balance the pH to the range of 7.6 to 7.8.

And that's how it went - 2-3 days to go green and cloudy, and 2-3 weeks to clear up! (almost 4 weeks actually!) He shocked the pool until he ran out of shock. Added Clarifier, but overdosed it, adding 3 doses in 24 hours. I suspected Phosphates from poor drainage during heavy rainstorms, so I recommended treating the pool with PhosFree. Afterwards, the sand pool filter was having difficulty clearing the pool, so he shocked the pool again (third time). Still, the color and clarity was not what it was pre-algae bloom.

I suggested Aluminum Sulfate, known as Alum, to improve the sand filter effectiveness. It's also used as a flocculent when added to the pool, but then you have an hour of slow vacuuming to waste. Instead of adding 4-6 lbs  to the pool, he added 12 oz. to the pool filter, by pouring it into the skimmer with the pump running. This forms a layer of particle trapping jelly on top of the sand bed, and improves the filtration temporarily.

After 24 hours, the filter pressure builds up and the alum and cloudy water particles are flushed out with backwashing. He retreated the following evening with another 12 oz. of Alum, and added a dose of clarifier (also on my recommendation), and the next morning the pool looked beautiful once again.


- Rob