Your guide to controlling algae in your pond
If you’ve ever seen green slime in stagnant water, you’ll know that algae can be a real problem for our ponds! Some types of algae do have their place in a healthy pond ecosystem, but we need to be sure that we keep the growth under control. There are various ways to do this and one method is to introduce some pond life that eats algae. Read on to find out more about pond algae and learn how to stop it getting out of hand.
What’s the problem with pond algae?
Pond algae can cause problems by reducing the oxygen levels in the water, which is harmful for fish and other pond life. Too much algae in the water can also block light from getting to other plants. What’s more, some species such as blue-green algae can produce toxins that are poisonous to wildlife.
Blue-green algae are always undesirable in a pond, and another unwelcome guest is filamentous algae (or ‘blanket weed’), which is harmless in small quantities but can easily get out of control. Another type of algae called planktonic algae is an essential part of the pond’s ecosystem though: these tiny single-celled green organisms form the basis of the food chain. So, for pond owners, the goal is to prevent the harmful types of algae from taking over and to make sure that the useful algae is present in sensible quantities.
What makes pond algae grow too much?
Various factors encourage the growth of algae, and how important these are will depend on the type of fish pond you have. Too much light, increased levels of nutrients such as phosphates and nitrates, and the absence of algae-eating animals can all encourage algal growth. Temperature also plays a role, with most algae growing enthusiastically in the late spring and summer but some species preferring the autumn, winter or early spring.
Do pond fish eat algae?
One way to keep algae under control is to stock your pond with creatures that eat it. Some fish will nibble away at algae, whereas others won’t touch it at all. So, what pond fish eat algae? There are many types, some of the most common of which are goldfish and koi. These fish don’t really have enough of an appetite for algae to bring high levels right down, so for larger ponds or more severe algae issues they will generally need some help.
Otos and plecos are two species that are more keen on algae and they love to hoover it up from the pond floor. Otos are tiny at a couple of inches long, whereas plecos can grow up to two feet! When you’re choosing your algae-eaters, consider the amount of room available in your pond and also remember to factor in the temperament of the fish and check they’re compatible. Plecos are generally docile but the larger ones could potentially cause harm to smaller goldfish, while large koi may be a threat to small otos.
While goldfish and koi can survive the winter outdoors if you make sure the pond surface is not covered with ice, many other algae-eaters are less hardy and may need to spend the winter indoors. Plecos and otos don’t cope well with low temperatures, and other algae-eaters such as mollies and guppies also need to move inside in cold weather. Loaches are algae-eaters that are happy down to lower temperatures but they will still prefer to move indoors or benefit from a pond heater in the depths of winter.
What else eats pond algae?
So, what eats pond algae other than fish? Snails are well-known for their taste for algae and they also help get rid of uneaten fish food. However, they can multiply dramatically and if the population gets too big, they can disrupt the ecosystem by nibbling through your pond plants and contributing to pond waste when they die.
One species of snail that doesn’t tend to get out of hand is the Black Japanese trapdoor snail. These are hardy enough to overwinter and they’re unusual for snails in that they don’t produce huge clutches of eggs – just a few live young at once.
Adjusting your pond’s ecosystem to prevent algae building up
As well as adding algae-eaters, there are lots of things you can do to keep algae under control in your pond. One is to consider the plant life – healthy oxygenating plants within the pond will compete with the algae for nutrients as well as helping to keep the water conditions healthy for your fish.
Sunlight encourages algal growth, so it also helps to make sure some of the pond is shaded. Shade could either come from something above the pond, such as a tree, or from floating leaves of water plants.
Since excess nutrients in the water support the growth of pond algae, it’s also important to avoid overfeeding fish (see our guide on how to feed fish to learn more) and consider the source of any groundwater that drains into your pond. An excess of fish waste can have a similar effect on algal growth, so make sure your pond is not overstocked and that the filter is adequate for the size of the pond and the number of fish.
Other strategies to control pond algae
A traditional strategy to stop algae problems developing is to add a small bundle of barley straw to the pond in early spring and autumn – the idea is that when the straw decomposes in water it produces substances that inhibit the growth of algae. The success rate with this method is variable but some people report it works well. However, it’s important only to add a little straw as too much can affect the water quality.
More modern control measures are also available, including UV clarifiers and pond vacuum cleaners. These can be very useful but many pond owners find that it’s possible to keep algae under control by adjusting the ecosystem to restrict algal growth to a reasonable level.