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The Basics of Pond Algae - Introduction and Treatment Guide

Pond Algae Problem


A beautiful pond makes a fantastic centrepiece in any garden. But at some point or another, your pond will most likely experience an algae problem, with many factors contributing to algae bloom.

Understanding what causes pond algae and how to get rid of it is key to maintaining a healthy pond. Thankfully there are a number of simple methods to help you keep the algae in your pond to minimal levels.

What is pond algae and what causes it?

Algae is a plant that is caused, like all other plants, by a combination of photosynthesis and excess nutrients in the water. 

Common causes of pond algae in the UK include:

  • Fertilizer runoff from your garden
  • Plants or leaves falling into the water and decomposing
  • Lots of sun without much shade
  • Common pond debris such as fish faeces and sludge buildup

Pond algae can also be caused by using tap water to refill a pond after cleaning. It’s always best to use either rainwater or tap water that has been left to stand for at least 24 hours to allow the chemicals to evaporate. If using tap water we also recommend using a natural dechlorinator.  

Algae Blooms

It is worth noting here that algae blooms are an entirely natural and normal occurrence in ponds of all sizes. Usually flash blooms happen in Spring as a response to seasonal changes, which then quickly die back down again within a few days. So if your pond is experiencing an algae bloom, don't panic at first. 

Algae is an incredibly important part of aquatic ecologies, and although it can appear to be a huge nuisance to our prized pond spaces, we don't often give it the credit it deserves.

A certain amount of algae in the pond is a very healthy sign, and having algae present can contribute significantly to the health and wellbeing of the pond. Moreover, algae is an incredible organism that displays a host of fascinating biological qualities.

So before you set out to eradicate your green neighbours, make friends and just remember that it's all about the balance.  

Different types of pond algae

Your pond may display algae in a number of different ways. One of the most common is a ‘pea soup’, where the water of your pond becomes murky with a green/brown buildup. 

You might also be able to see floating pond scum, in the form of blanket or string algae. These are types of growth that can clump together around shallower parts of your pond, or around the edges. Sometimes they can end up engulfing certain plants as well.

As algae is a living plant, like your pond plants, it consumes oxygen and nutrients from the water. Left for too long, it can start to damage your existing pond life including fish and other ornamental pond plants. 

So, if you spot pond algae, it’s best to look at treatment solutions as soon as possible. However, prevention is always the best form of 'treatment' and catching it early will save you potentially bigger problems down the line. 

Removing algae

Pond Algae Pea Soup Problem

‘Pea soup’ algae will need treating with some of the holistic solutions below. If it is particularly bad, a chemical treatment may be required.

However, we think by removing excess sludge and taking care of some of the fundamentals, you should be able to reduce the green water algae. It’s best to leave sludge removal until either the Autumn or late winter/Spring months to avoid disturbing the ecosystem and life in your pond.

With string algae, you will simply need to remove it with a pond net or by hand. It’s a messy job, but an essential one to keep your pond healthy.

How to prevent pond algae

There are a number of chemical solutions for treating and preventing pond algae. However, we think that using natural methods whenever possible is the best for your pond, and the plants and animals that call it home.

1. Check your filters

If you have a pump or filter in your pond, make sure to check it for any blockages. When pumps and filters stop doing their job efficiently, this can cause a quick buildup of nutrients and contribute to the conditions for algae to grow.

2. Fish and plant balance

Pond plants and fish can both work to reduce the algae in your pond. After all, plants rely on the waste in the water for nutrients, and the fish rely on the plants for their habitat too.

However, if you get the balance wrong, this can cause an overabundance of nutrients which can cause algae bloom. With too many fish, for example, the buildup of faeces can cause too much ammonia in the water which can in turn lead to problems with algae. It might take some time, but it's worth experimenting to get the balance just right.

3. Choose the right pond plants

Like algae, pond plants feed on the nutrients in your pond, so the right plants can provide competition for pond scum. 

Floating plants such as lilies and lotuses serve a double purpose in that they are hungry plants that consume nutrients in the water and they also provide shade for your pond. As these plants create a competitive environment in the water, this reduces the opportunities for algae photosynthesis, and thereby makes it harder for algae to develop. Ensure there is ample coverage over the water surface of your pond. 

Submerged plants such as hornwort and parrots feather also oxygenate the water, which helps to keep the water clean and clear. There are many excellent varieties of oxygenators to choose from, that not only act as a key component for algae prevention but are also indispensable for healthy pond habitats. 

4. Algae eaters

Most popular pond fish do feed on algae, including:

  • Pond loach
  • Koi
  • Common pleco
  • Goldfish
  • Grass carp

Pond snails also help to keep the algae levels down, as well as freshwater mussels. 

5. Remove debris

Dead plant matter, leaves and other bio-debris can all contribute to the build up of algae in your pond. Regular maintenance to remove dying plants will reduce the decaying organic matter in your pond and therefore give algae less chance to grow.

Preventing algae from falling into your pond with a net can also be very useful, especially in the Autumn months.

6. Shading

In the spring and summer, when the sun is falling on your pond, is when you are most likely to see algae blooms in your pond. If your pond is in direct sunlight, you can help to minimise the exposure to sun with marginal plants such as sweet flag and sedge. As mentioned before you can combine marginals with water lilies to increase the surface coverage of the water.

These plants also serve to filter the water, protect against runoff from your garden, and offer a great habitat for pond dwellers such as frogs and newts. 

7. Refill with rainwater

During the summer months especially, you might spy the water level of your pond dropping. Try to avoid using tap water, unless it has been left to stand for at least 24 hours.

Ideally, use rainwater to refill your pond. In the UK, harvesting rainwater throughout the year isn’t a problem, so invest in a butt or other water storage to top up your pond when needed.

8. Pond Clear

Pond Clear is a very useful method to prevent pond algae and is completely natural. It comes as a basket containing a barley straw mix, which when submerged under the water, slowly decomposes over a period of months. The chemical reactions that take place as a result of decomposition via sunlight slows down the growth of algae. Its efficacy is usually improved in aerated ponds with higher oxygen flow. Liquid barley solutions are also available, which can help as a temporary respite from growing algae problems. 

Clear Water pond

Taking care of your pond

Regular maintenance and cleaning is the best way to minimise excessive algae in your pond. As we’ve mentioned, algae is a common occurrence, especially when the sun is shining, so keeping it under control is the best way to avoid it becoming a problem.

Remember to clean your pond in the Autumn or Spring, and remove excess sludge or dead plants. And make sure your pond plants are up to the job of keeping your pond clean and maintaining it as the beautiful feature it should be.