Algicides, often called algaecides, are a vital ingredient to control the nuisance of algae in swimming pool and spa water.

algae in swimming pools

What are algae and why don't I want them in my pool?

Algae are microscopic single cell plants. They exist in water from fish tanks to reservoirs. Whilst not directly harmful, they are undesirable in swimming pools as they discolour the water, usually green, which is most univiting to the swimmer. Eventually the water can become opaque and dangerous in that a bather in difficulty may not be seen. The pool structure will also be slippery if algae are attached to it.

As they are plants, they thrive in the same conditions. They require Nitrogen, Phosphorus and Potassium, the same the plants in your garden. They also like sunlight as it is essential for photosynthesis. This is where chlorophyll, which causes the green colour, utilises light energy and converts carbon dioxide into organic compounds essential for growth and gives out oxygen.

Nitrogen is most likely introduced into the water by bather contamination. Shock treatment, itself a good method of killing algae ensures that some nitrogen is released as a gas rather than staying in the water as an assortment of nitrogen compounds.

Phosphorus is introduced into the water in a variety of ways

  • In the water supply where small amounts are introduced into the water by the supply company to prevent corrosion
  • From water softener additions, such as sodium hexametaphosphate
  • From scale control chemicals, containing phosphonates
  • From cleaners containing phosphoric acid
  • From accidentally introducing garden fertilisers into the water from surface run off or sprays. This will give you Nitrogen and Potassium too!

The phosphorus containing chemicals above will all eventually revert to orthophosphates which is what algae really like.

Of the three nutrients, potassium is the least likely element to be found in pool water.

I've got algae, how do I eliminate them

Calculate algicide dose rates for a pool of known volume

How do I prevent algae getting into the pool?

By creating conditions that algae don't like

Regulatory Information

For full information, download an algicide safety data sheet

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