Purple Pool Stains

Purple stains in pool

If you do a web search for “purple pool stains”, “pool purple ring”, or “pool purple algae,” a variety of results are displayed, each with a different reason for the purple stains. These range from high cyanuric acid, high copper levels, high magnesium levels, alkalinity problems etc., etc. I checked with our own resources and found a single reference to purple stains in the 3rd edition of the “APSP service tech manual” in chapter 8 section 28:

“Manganese produces a black-brown-purple cast and may not be noticed until extra chlorine is added or until it is necessary to raise the pH.”

Note: APSP stands for the “Association of Pool and Spa Professionals” and they are the association that produces a detailed manual covering swimming pool maintenance. Find more information here: ( APSP Website )

But I still am not 100% convinced what is causing this. We do around 100 pool/spa’s a week with little in common so we have a large example set to test what works and what does not. Just from my own experience I would guess that alkalinity/pH problems are causing the purple stains.

While the origin of these stains in pools seems to be debated among pool professionals, the method of removing them is known. The key to purple stain removal is with the use of an acidic product. Muriatic acid is the strongest form of acid available that is intended for use in swimming pools and most spas. Figuring out how to properly apply the acid safely without causing any damage can be difficult. Stains can appear on plaster, sides of vinyl, on automatic cleaners and cleaner hoses, and anywhere else at or under the water level.

Applying acid

The difficulties of this step come from the use of the acid itself. Muriatic acid will stain cement, corrode metals, burn skin, kill plants, produce a toxic cloud when expose to the atmosphere as well having other dangerous attributes. This is why I would recommend that most swimming pool owners hire a professional for any purple stain removal. Only a trained professional should attempt to handle acid. The probability of something going wrong is high when working with such a dangerous chemical for the average pool or spa owner.

Note: This is a guide of how we would handle this situation not a guide on how you should do it.

Items needed:

Muriatic acid (1 or more gallons)

A chemical sprayer (I found this one at Home Depot)

Chemical grade gloves (not gardening or dish gloves)

A scrubbing brush

A connected water hose to rinse whatever the acid is being applied to

Applying acid to an object:

There are times when only an object in the pool has a purple stain; some common objects that stain easily are automatic pool cleaners and cleaner hoses.

Note: Using acid, regardless of whether it has been diluted or not, on any object could damage or discolor it. Only a trained professional should attempt to handle acid.

When applying the acid, do not spray over dirt or concrete and remember that only a small amount of is needed (the chemical sprayer should have a mist option or mist on default).  Rinse immediately after a light spraying.

A handheld brush might be needed to get a small amount of acid spread out equally around the object.

After removing all purple stains, a swimming pool metal remover is recommended since the water was left in the pool/spa. One example of a chemical metal remover is “Metal Free,” which available from Natural Chemistry (if this was caused by a metal).

This will reduce the chance of future staining.

Applying acid to a surface:

Note: Using muriatic acid, regardless of whether it has been diluted or not, on any pool or spa surface could damage or permanently stain the surface.  Only a trained professional should attempt to handle acid.

The goal here is not to acid wash your pool/spa; it is only to remove the purple stains.

In a standard acid wash, the first step is to use just enough acid to try and gently remove the stain, leaving the pool surface intact. If that fails, a greater amount of acid is used that will essentially remove a “layer” of the surface (i.e. a layer of plaster is removed from the pool surface after being dissolved by the muriatic acid which exposes a brighter non-stained area in its place), This is why only someone with experience in applying acid should perform this step since going too far will permanently damage the surface.

The areas that have purple stains must be above the water line to be removed. Begin by draining the water down in the pool/spa. The more water drained, the better.

(If you are unsure about draining a pool/spa, see our past write up here)

Only a small amount of acid is needed to remove these purple stains. Use the misting option of the sprayer.

Having a brush to scrub the area helps since it minimizes the amount of acid needed, which is good for the plaster.

Rinse the acid off with a water hose.

Repeat for all areas with purple stains.

Now refill the pool/spa.

The advantage of this procedure is replacing the water in the pool/spa. This is the most efficient method for removing or greatly reducing the amount of metals in the water and starting over with the alkalinity/pH that will reduce the chance of future staining.

And done.

Note: Again, this is a guide of how we would handle this situation not a guide on how you should do it.