If you have a lot of algae it means there's a problem with your pond's ecology. You can choose to treat the symptoms or the actual disease. We'll tell you how to do both. But first, the basics.

Introduction to Algae

Algae is a plant, and like any plant it needs nutrients and sunlight to thrive and grow. Garden ponds are small, so the proportion of nutrients to water to sunlight can quickly become far more optimal for algae growth than might be the case in nature.

The nutrients that algae need to grow come from nitrates in the water. Nitrogen is routinely used by gardeners as a fertilizer to promote leafy growth in plants. In nature, high levels of it are found in animal excrement and in decomposing organic matter. This explains why people with fish also tend to be people with algae.

As for the second algae ingredient, we all know where sunlight comes from. But while you may not be able to control how much sunlight hits the surface of your pond, you can control how much of it penetrates the surface.

So, the disease is algae and the causes are too many nutrients in the water and too much sunlight. Will you treat the symptoms or go after the disease?

Algae Solutions

To treat the disease you have to remove its causes. If you have fish, give most or all of them to a friend and don't feed those that remain (they will get plenty to eat from bugs and other sources--you'll see). Next, make sure that you don't have a build-up of organic material in the pond, like leaves or crab apples. But don't drain and clean the pond thoroughly like you would a fishtank, this can itself cause algae problems. Also, check for run-off. Lawn and other fertilizers contain Nitrogen and other chemicals that can cause algae growth.

Once you have eliminated the sources of excess nutrients in the water, you need to tackle the sunlight problem. Most garden ponds (those under 30x30 feet) should have 50-70% of their surface covered with floating plants like water lilies, floating heart and duckweed. The leaves of these plants will keep sunlight from entering the pond. If your pond is in part-shade you can make do with 25-40% coverage.

In addition to filtering sunlight, water lilies also act as a natural filter, pulling large amounts of Nitrogen and other nutrients out of the water. In fact, water lilies are routinely used to treat contaminated wetlands because they are known to absorb things like heavy metals as well.

With floating plants you can turn your vicious algae cycle into a virtuous water lily cycle.

Treating the Symptoms

It's not necessarily a bad thing to treat the symptoms, but it usually requires more work and expense. People who will need to treat the symptoms are almost always those who love their nitrate-producing fish and don't want to part with them.

We still recommend using water lilies to block sun and absorb nutrients, but it's virtually certain that you will also need a sturdy Bio-filter to help remove nitrates, and possibly a UV filter.

Biofilters basically consist of media to grow bacteria. These bacteria consume nitrates in the water. They will help with all types of algae. UV filters work very well if you have the kind of persistent "pea soup" algae that causes cloudy water. However, they won't treat string and other types of algae.

Blithestone sells both bio and UV filters. In general, we're skeptical of any type of chemical filtration. Water gardeners should also beware of gimmicky products that promise quick solutions when it comes to algae--we've found that there are quite a few out there.

Further Reading at Blithestone

More information on algae prevention can be found in the Pond Guide in the main menu and in the drop-down menu--see in particular the Ecology sections. We also discuss algae in different editions of the Water Gardening Journal, and in our About Water Lilies section.

Information on filtration strategies can be found in our About Filtration section.