Algicides, often called algaecides, are a vital ingredient to control the nuisance of algae in swimming pool and spa water.
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
- Shock dose by raising the free chlorine level to above 10 parts per million if the water is a greenish colour and up to 25 parts per million if you can't see the bottom. This treatment should be done whilst bathers are prevented from entering the water. More information about Calcium Hypochlorite and shock dosing
- Brush colonies of algae from the structure of the pool. The more virulent types such as black and mustard varieties may need scraping off. Vacuum and remove these residues by filtration
- Introduce an algicide into the water. This will most likely be in the form of copper chelate which slowly releases copper into the water, hence the term "longlife algicide" or if you must "long life algaecide". Aim for an intial free copper level of 0.8 p.p.m and maintain at 0.4 parts per million. Don't overdo it, otherwise copper may start to deposit on metal surfaces and it may turn your hair blue! The alternative non copper type will probably be a quaternary ammonium compound or a variation known as a "polyquat"
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
Swimming Pool and Spa Chemicals website
- Keep the pool cover on - Obviously not always practical
- Have an indoor pool without too many windows
- Keep the sanitiser level up at all times. A regular testing and maintenance of the chlorine level is essential
- Restrict use of phosphorus containing chemicals to spa baths where the water is regularly changed, or indoor pools
- Avoid getting tile and liner cleaning products in the water
- Consider treatments like the use of sodium tetraborate which interferes with photosynthesis. Don't use the potassium salt as you will be introducing potassium which is a nutrient
- Consider using poly aluminium chloride flocculent if your water supply contains phosphates. Ask your water supplier, they should be able to tell you
- Chemical Name (or "contains"): Because they are usually "preparations" (mixtures of things) the labels of algicides will probably say contains a copper or cupric something or other.
- UN Number 3082 Environmentally Hazardous Substance, Liquid, Not otherwise specified.
- Class 9 (Not usually found on small packs)
- EC Number - Not applicable to mixtures
- CAS No. - Not applicable to mixtures
For full information, download an algicide safety data sheet
Return to top of page