Now that the long winter is finally ending, pool owners are looking forward to pulling off their covers.

One of the strangest experiences a pool owner can have is to discover upon opening that the pool has a purple crystal-like stain on its surface. Service professionals have known about this phenomenon for years, and commonly refer to it as purple haze.

Purple haze is not, contrary to popular belief, an organic bacteria or an algae issue. The problem is actually the result of improperly balanced water chemistry combined with a mineral issue. The purple staining is caused when the pool water has formed something called copper cyanurate.

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Copper cyanurate occurs when excessive levels of cyanuric acid combine with non-chelated copper that is present in the water. This problem is most common in pools that have cyanuric acid levels measuring above 100ppm.

This previously rare phenomenon seems to be on the increase these days. The use of cheaper, lower quality copper algaecides at pool closing (which are typically sold at mass merchants) appears to be a major reason for the spike.

Ironically, colder water like we have in the spring aids the reaction between the copper and cyanuric acid to form these purple crystals. Copper cyanurate is not easily scraped, scrubbed or removed.

Dealing with a Purple Pool

If you do have to deal with a purple pool, there are steps to correct the problem. The first step is to lower the cyanuric acid level of the pool water to around 30ppm by performing a partial drain and refill. When you lower the CYA levels by partially draining and refilling, you also happen to be reducing the copper levels in the water as long as your source water does not contain copper. This is a good time to add a non‐phosphate metal control product to deal with the copper that still remains. Once the CYA levels are in line, adjustments need to be made to the pool water’s pH and total alkalinity.

It is possible, through normal stain removal procedures, to eventually remove the stains, but typically it requires three times the normal amount of traditional stain removal chemicals and a lot of time.

The 3Ps to Prevent Purple Pool

Prevention is truly the preferred method for dealing with the purple haze. Keeping the cyanuric acid levels below 100ppm (and ideally in the recommended 30-50ppm range) is the first step of prevention you should consider.

You should also consider reducing the amount of stabilized chlorine that you use during the pool season if you tend to see your cyanuric acid levels end up on the higher side. The best way to do this is by using alternative shocking products that will not increase your stabilizer level such as calcium hypochlorite or liquid chlorine.

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Avoiding the use of poor quality copper algaecides at closing is also a must. Copper algaecide is a fantastic off-season algae prevention product when it is properly formulated. But if you are encountering this problem because your customer used a lesser grade copper algaecide, remind that customer that pool professionals stock versions of copper algaecides that are designed to provide protection against unintended staining like copper cyanurate. It’s important to remind your customers that some mass merchant copper based algaecides are nothing more than stains waiting to happen.

Chris Marcano is National Director of Training and Education at Natural Chemistry, Overland Park, Kansas.