We hear this question a lot. While there are a number of potential causes that you might have cloudy pool water, there are several key causes that create the significant majority of cloudy water issues. I’ll address each of these most common causes individually. Keep in mind it is possible on some occasions that the cloudy pool water could be related to more than one cause being in play. I will assume you already have chlorine in the swimming pool water and the ability to utilize a circulation and filtration system as a remedy.
1.) Cause #1: Did you shock or super chlorinate recently?
The most common type of shock or powder-based chlorine is calcium hypochlorite. “Cal-hypo” as we call it in the pool industry is highly basic or high on the pH scale. This high pH means it causes cloudiness when introduced to pool water. Basic things are usually cloudy. Think of cow’s milk (basic, high pH) compared to lemon juice (acidic, low pH).
Remedy: Run your filter continuously to turn over the water and clear it. With a Diatomaceous Earth filter this may only take a few hours since it has the finest level of filtration. A cartridge filter might take 2-5 times as long. Sand filters might take multiple days. A trick for a sand filter is to add a couple of 1lb coffee cup sized scoops of DE powder to them to help them filter a little finer.
Note: With all filters be sure to monitor the pressure and make sure you backwash or clean them if it gets 5-10psi higher than the clean filter pressure for your system. With DE filters we usually shut down a system if we know the pressure has risen above 30psi to avoid damage to the filter internals. Cartridge filters can sometimes be configured to run clean at 30psi. In this case I would strongly suggest cleaning them out before the pressure gets close to 40psi.
2.) Cause #2: Do you have a combined higher than average level of pH, Total Alkalinity, and Calcium Hardness?
While each of these chemical items might be within an acceptable range the combination of each of them pushing the high end of their range can mean the water is out of balance. This is even more likely to be a factor if the water temperature is high. When the combination of these three chemical items and the water temperature is on the high side we have the potential for calcium to precipitate as scale and to make cloudy water.
The index used to describe the relationship of pH, TA, Calcium and Temperature is called the Langelier Saturation Index (LSI). If the Langelier Saturation Index has a positive reading then the swimming pool water has the potential to scale.
I want to reiterate that you can have each chemical component within an acceptable range and yet if they are all pushing the high side of the acceptable range the combination can be to create water that is pushing calcium out which more easily causes both cloudy water and scale. The good news is you can re-balance your water by adjusting the pH. You can use this widget to calculate the correct pH for your pool water.
As a quick reference here are the ranges we manage towards for swimming pool service in the Dallas, TX area:
Chemical Industry Acceptable
pH (Potential Hydrogen) 7.2 – 7.8
TA (Total Alkalinity 80-120 ppm
Calcium 200-400 ppm
As a side note we like to keep the pH at or below 7.5 as much as possible. This provides for the chlorine in the pool to be more effective. For more on this topic see my blog post on how high pH affects chlorine effectiveness.
Remedy #1: If your pool water is prone to precipitating calcium you can almost immediately achieve balanced water by adding acid to lower the pH to the recommended level per the Langlier Saturation calculator above. Adding acid will also lower the TA slightly. However if you are compensating for high calcium via pH that has to always be at the low end of the range I would go ahead with Remedy #2.
Remedy #2: If your calcium is above 400ppm I would recommend a partial drain to lower the level. The new water from the tap will have an effective calcium level of almost zero. This means if you do a ½ drain on a pool with a calcium level of 500ppm that you will be reducing the calcium to 250ppm. Once you have added water make sure to rebalance all your chemicals.
3.) Cause #3: Did you have a storm recently?
Storms bring pollen, dust, sand, dirt, leaves etc into the pool very quickly. All this debris will begin to immediately be broken down by the chlorine in the swimming pool water via oxidation. Oxidation turns small particles and debris into a dust or powder that can cause cloudy pool water.
Remedy: (same remedy for cloudy water due to shock) Run your filter to clear your swimming pool water. DE filters will clear the fastest. Refer to the remedy above for shocking for more details on how long utilizing the filter will take and a trick to make a sand filter clear the pool water faster.