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Algae and green water in ponds

Blanket Weed

Blanket weed, often called string algae is frequently a problem in ponds. It is a form of algae that attaches itself to solid objects within the pond, such as the sides of the pond, plants and pumps (which can become clogged). Once attached the blanket weed will form into green strands that trail out into the pond, usually following the flow of the water and often seen on waterfalls. It is an unpleasant sight in a pond and can become so prolific that the fish can become entangled in it and die. The spores for blanket weed are naturally found in tap water and it can also be introduced when adding new plants that have not been cleaned. There are two commonly seen forms of blanket weed, a dark green variety and a brighter green variety, which although similar in behaviour does tend to break up more upon touch and is somewhat slimy.


Combating these dreaded weeds, can often be a bit of a trial and error scenario as there are many different parameters which affect it such as the amount of sun a pond receives each day, water chemistry, fish stocking rates, filtration effectiveness etc. There is a great range of products available from many different manufacturers and they usually claim themselves to be the best. Unfortunately in reality every pond is different so a product that works for one person will not always be the magic solution for someone else. Always read and follow the products instructions before usage and also remove as much weed as possible before treatment as dead weed will rot and pollute the ponds water. Below are some of the more effective methods for combating Blanket Weed. 


Often very strong and usually effective against Blanket weed and suspended green water algae but they will normally kill any submerged plants such as Lilies, Oxygenating plants etc. and if overdosed can even kill the fish and amphibians. These can be good if dosed correctly for plant free ponds such as Koi ponds. They come in both liquid and granular forms.


Plant Friendly Treatments
These are much safer and do not normally present a danger of overdosing, while still being very effective, but can be expensive to use considering their frequent dosage requirements. They usually alter the water chemistry so as to produce an environment that is not conductive for blanket weed and do not destroy other submerged plants but some can slow their growth rate.


Electronic Weed Control Systems
Yet again many different brands available and often highly praised but they must normally be installed on pipe work which is at maximum 1 ” (32mm) in diameter. They work by wrapping two or more antenna wires around the out side of a section of the ponds pipe work and produce a frequently altering signal, which alters the water chemistry. These units are safe to use with plants and fish but on some ponds they can be totally ineffective.


Biological Treatments
More recently a range of biological treatments have come onto the market, produced by several manufacturers. These claim to introduce bacteria, which either feed on the blanket, weed or use up its food source by reducing the levels of nitrate and phosphate. Treatment with these products is claimed to last for the entire season or even longer, which is much more cost effective than the other methods available. We are currently testing what appears to be the best of these products on some of our own ponds and initial results are looking very promising but, until we have finished our tests, cannot recommend or advise on their full effectiveness etc.

Green Water

Algae are a minute single celled plant that can grow rapidly when in contact with sunlight. Even if drawn straight from a domestic tap pond water will contain millions of these single celled plants and they are so small that you cannot see them with the naked eye. Once in contact with sunlight they can very quickly grow large enough to become visible.

Although algae are often the reason for green water they can also cause brown water conditions, as there are many species of algae. Within a pond the natural fertilisers, fish waste and decaying plant matter, serve as an excellent food source for the algae.


Algae can coat most parts of the pond, including the ponds surface, and the ponds submerged plants with a fine green slime, but it is the suspended particles that cause the colouration to the ponds water. In heavily stocked ponds, with no plant life and no filtration system, the water can become so green that even large fish just one inch below the surface can be impossible to see.

As the algae are suspended in the water the deeper a pond is the more exaggerated the green water will appear to be.

The fish within our ponds do not object to the coloured water caused by algae as it provides them with a good source of cover from predators, but if it becomes excessive it will then deprive them of dissolved oxygen. On the other hand most of us create a pond so that we can see and enjoy the beauty of the life within it.

Below are some of the more effective methods for combating Algae. Always read and follow the products instructions before usage





Pond Plants
Ponds with plenty of plants do not often suffer from serious Algal blooms as the plants are competing for the same food source as the algae. Lilly leaves in particular can help to shade the pond from sunlight. Adding Plenty of Oxygenating plants often helps to reduce algal blooms within a pond, but pond plants (especially oxygenators) also spread rapidly and can soon become just as big a nuisance as algae.


Basic Liquid Treatments
These are usually mixed in a bucket or watering can prior to being distributed evenly over the entire surface of the pond. They generally work by slowly sinking and clumping the algae cells together, which are drawn down to the bottom of the pond where the algae will die, probably from lack of sunlight. If you have a filtration system fitted most of the sunken algae will become trapped within the filter, which must be cleaned soon after. If you do not have a filtration system fitted this method leaves all of the dead algae to rot at the base of the pond. Further algal blooms can soon occur.


Often very strong and usually effective against Algae and Blanket weed but they will normally kill any submerged plants such as Lilies, Oxygenating plants etc. and if overdosed can even kill the fish and amphibians. These can be good if dosed correctly for plant free ponds such as Koi ponds. They usually come in a granular or liquid form.


Barley Straw
Supposedly this was discovered by accident when a farmer dropped a bale of barley straw into a pond that had been green for years, only to find that it then cleared soon afterwards. Modern treatments often have other additives and can be purchased in three ways, as a pouch containing barley straw, as a liquid containing the effective substance from within the barley straw or as an impregnated foam pad. These treatments are effective and they usually require a month to start working and regular monthly replacements but do not appear to adversely affect the pond.


Caution; Never look directly at an illuminated UVC bulb as this can cause serious damage to your eyesight, most Models have either a small viewing window or specially treated transparent hose tails so that you can check that they are illuminated. Always read the instructions fully prior to installation.

These are the most effective and permanent form of control but you must replace the light bulbs every spring regardless of if they appear to give off light or not. The light bulbs can be quite expensive but these units really do work well so long as you have chosen the correct sized unit for your pond and they must be used in conjunction with an adequately matched pump and filter.

They work by passing the pumped pond water around a clear quartz tube which encases a light bulb that emits ultra violet radiation and this makes the algae swell and clump together so that it is large enough to be trapped within your filter. These units do also have a mild sterilizing effect on the water. It is sometimes necessary to turn a UVC off during the use of pond treatments (always check the treatments label).