how to control algae in your pond

The pond environment is a complex one and can require careful maintenance to remain healthy, especially when a pond is new. A build-up of algae is unsightly and is detrimental to your fish stocks, also preventing you from seeing them. Fortunately there is much you can do to prevent algae from forming and to rid your pond of this unsightly green film.

What causes algae?

Algae need sunlight, ammonia and oxygen in order to prosper. It's crucial that you're aware of the levels of ammonia, nitrite and nitrate in your pond and you can assess these using a testing kit. The test kit will give you a good idea of what is causing your algae problem if you have one. Alternatively it will warn you when the conditions are ripe for algae growth.

Pond Pumps and Pond filters

Pumps and pond filters are vital for water quality. Your equipment must be suitable for the size of your pond and should be carefully maintained. Algae growth is often the result of poor filtration. This could be because the filter doesn't have sufficient capacity for your pond and so cannot process all of the waste material. The pond filter should be able to circulate all of the water every two hours. Your filter media contains nitrifying bacteria which break down ammonia. But the bacteria need time to mature. If your filter does have the correct capacity then any algae issues could be due to insufficient levels of bacteria in the filter. You can use cultures of bacteria to improve the situation. If your filter and bacteria are in order then your problem could be due to overstocking. Removing some of the fish could do the trick. Your pump should be kept free of debris as this will impede your system. Clear your filter sponges of debris and then wash them in pond water. Do not use tap water as this can kill your good bacteria.


Algae could be prospering in your pond because sludge has accumulated at the bottom. This will be fish waste, uneaten food and decaying plant matter. The process of decay produces ammonia which again, promotes algae growth. Use a pond vacuum to clear the sludge and consider fitting a pond net to prevent leaves from falling into the water in autumn.


The fertilisers and other chemicals that you use in your garden often feature ammonia. Surface run off can cause this to end up in your pond water. Check the products you are using and then change them if necessary. The process of fertilisers causing water pollution and in turn encouraging growth in plant life such as algaeis called eutrophication.

Excess Food

Uneaten food will eventually end up at the bottom of your pond where it will rot and release ammonia. Make sure that you are feeding the correct food for the species that you keep and feed only what the fish will consume in two minutes. Clear uneaten food from the water after this time.

The Sun

If your pond is exposed to the sun all year round then algae will prosper. You could consider relocating your pond to a more shaded area, planting large shrubs and trees to provide shade or even constructing a hanging garden.

Algae Treatments

Algae treatments are available in biological and chemical forms. These speed up the breakdown of algae. You could also think about adding a UV clarifier to your filtration system. These shine UV light onto the water prior to it entering the filter. The process results in algae particles clumping together and being trapped in the filter where they are broken down. For a more natural approach, adjust your planting to feature ammonia feeding species. These will help to starve the algae.

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