Controlling Pond Algae

Out of control pond algae looks like thick green pea soup, feels like slime, and smells of stagnation.  Not only is it unpleasant, it is harmful to a pond and its inhabitants.  It uses up too much oxygen for goldfish, wildlife and plants to thrive, and may eventually result in their death.

There are a number of practical steps that can be taken to control pond algae, most of which form part of basic pond management anyway.

Pond Construction

By following the steps outlined in the earlier blog post Build the Perfect Goldfish Pond, pond algae will already be less of a problem.  By making sure the pond is not positioned where a lot of debris will fall into it, there will be less nutrients for the algae to use for growth.  Using a black liner to absorb sunlight rather than reflect it into the water also deprives algae of the conditions it likes.  If the pond has sloping sides leading down to a bottom level, it makes the job of collecting any debris that does fall in easier, as it will be more localized.  Finally, have a pond that is deep enough to prevent strong sunlight penetration, and large enough to reach its own level of ecological balance will also work as an excellent preventative measure.


Plants can be used to provide shade over the water where long periods of direct sunlight are a problem.  Water lilies are the obvious example, but there are many different types of aquatic plant that can be used.  Plants also have the added effect of using up the nutrients that algae would otherwise thrive on.


It is a fairly obvious point, but floating plant debris should be removed from the surface of the pond as often as it appears.  If it remains long enough to sink to the bottom it starts to produce the nutrients that fertilize the algae, and is more hassle to remove.


If fertilizers are used on surrounding lawns, it is vitally important to prevent run-off from the lawn into the pond.  If the pond is at a lower level than any part of the lawn, some sort of barrier needs to be  in place that will direct contaminated water around the pond.  Garden fertilizers are designed to make green things grow very quickly.  They can cause disastrous algal blooms in goldfish ponds.

Barley Straw

When barley straw is placed in water it starts to decompose.  Chemicals released during the decomposition process are effective in stopping algal growth.  Bunch it inside a net – a couple of large handfuls should do for an average sized pond – and when it turns black its job is done.  Repeat the process as necessary to help control algae.  Barley straw extract is also available as a commercial product.


A skimmer is a mechanical device, powered by electricity or battery that removes floating debris from the surface of a pond.  There is no need for one unless manual surface cleaning doesn’t take place.

UV Filtration

Ultraviolet lamps can be purchased which are then placed in the pond water.  As the algae suspended in the water pass over the lamp they are exposed to UV rays.  These rays destroy algal DNA, and the result is usually a very clear pond.

Chemical Treatment

Anti-algal chemical treatments are available and offer a quick and easy solution.  Although generally not harmful to goldfish, they can be poisonous to frogs, snails and insects that help create an ecological balance and increase biodiversity in the pond.  As well as that, they are not always effective against all (over 17,000) types of algae.

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