Fiberglass Swimming Pool Staining Problem and Solutions

Last Updated: | By Barack James

Fiberglass Pool Stain(Image Source: Pinch A Penny)

Image Source (Pinch A Penny)

A fiberglass swimming pool is the easiest pool to maintain due to its smooth surface, which is easy to clean and kind of resistant to algae. 

However, when general cleanliness and chemical levels, especially Chlorine, pH, and Calcium Hardness, are not maintained properly, it is easy to find organic and inorganic stains in a fiberglass pool. 

Causes of Organic Stains in a Fiberglass Pool

1. Leaves and Pollen

Organic stains are caused mainly by leaves and pollen and appear green, yellow, and sometimes brown. Organic stains are easy to clean and can be brushed off using a normal pool brush. 

In some cases, an organic stain can be a little sticky and hard to brush off and might be mistaken to be an inorganic stain, but will eventually disappear on its own after a couple of weeks or a month and should not worry you a lot.

It is however important not to assume that you have an organic stain if it does not come out easily. To be sure that it's an organic stain, you can rub the stain with a chlorine tablet and if it loosens, that is an organic stain.  

To keep away organic stains in your fiberglass pool, make sure you clean your pool regularly by removing leaves and pollen that fall into the pool especially during the Fall.

2. Algae

Algae are also organic stains that come as a result of low free chlorine levels. As I said earlier, getting an alga in a fiberglass pool is not easy due to the smooth and coated surface that it comes with. 

However, this depends on how one maintains pool chemicals in the water, particularly the free chlorine level. Algae can be green, yellow, and black. 

To be sure you are dealing with algae (organic stain) and not a metal stain (inorganic stain), conduct an Overnight Chlorine Loss Test (OCLT) by adding chlorine into the pool in the evening and compare FC reading in the morning, if it falls by more than 2ppm in the following morning, that is an alga.

Green alga may hang loosely on the pool walls and sometimes floats on the water. Yellow or Mustard alga are sticky and may need lots of chlorine and brushing to come out. Black alga appears as blacks spots on pool walls and at the bottom of the pool. Here is more on how to Get Rid of 3 Types of Algae in Your Pool 

Causes of Inorganic Stains in a Fiberglass Pool

Inorganic stains are stains caused by heavy metals including Copper, Iron, and Magnesium among others. Stains are formed especially when chlorine is added into the water or when pH level escalates, oxidizing these metals. 

A light green stain indicates the presence of Copper and always discolors the water green. A Brown or yellow stain indicates the presence of iron, and a black stain is caused by Magnesium. Iron stains will always appear yellow on fiberglass pool walls and steps. 

These heavy metals may find their way into the pool when Well or Borehole water is used as fill water in the pool. 

Apart from the Well water, these minerals can be deposited into the water when one uses pool chemicals that come with metals such as Copper-based Algaecide, or as a result of corroding metallic fittings into the pool. 

The most common metal components we find in swimming pools are Copper and Iron. To clear green stains caused by copper and yellow/brown caused by Iron, we recommend Essential Values Stain Remover with the best chemical formula that is not harsh for fiberglass pools like other brands. 

To prevent metal staining in your pool whether Fiberglass or plaster pool, we recommend you avoid using Well water as your fill water. 

If you cant avoid using Well water, you can use a metal CuLator on your skimmer or pump buckets to remove metals before entering into the pool. Alternatively, you can add ProTeam Metal Magic into your pool to remove metals through the filter and thereafter clean your filter.



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