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Pond Algae – A Growing Problem

Pond algae is a common complaint among pond-owners, and whilst green sludge is the stuff of nightmares, a totally algae-free pond will always be a pipedream.  Because the truth is, there’s no such thing as an algae-free pond.

That’s because algae occurs naturally in water and it’s actually essential for a healthy and ecologically balanced pond.   But whilst every pond will have some algae growing in it, it doesn’t mean that you have to settle for dark, thick and unsightly garden water features.

With regular maintenance and attention, pond algae can be kept under control and to a minimum.

If you prefer clean to green, there are some basic algae facts that you should know which will help with maintenance and planning.   Perhaps the most important thing to understand about controlling the growth of algae is why it grows.  Once you understand what type of environment is conducive to the growth of algae, it will be much easier to know what to do in order to control it.

Basically, algae needs light and nutrients (nitrates and phosphate) to thrive.  It loves a pond where there’s debris at the bottom which results in loads of nutrients. It also loves food, so it’s particularly happy when your fish are being over-fed or your water lilies are getting a bit too much fertiliser.

It likes shallow water where the temperatures are warmer and it loves flat surfaces.  Most of all, algae likes it when the complex system in your pond is imbalanced, so to keep the green stuff in abeyance, the trick is to do the things that restore equilibrium.

Here’s a brief synopsis of what algae likes – and what to do about it:

Don’t overfeed fish or overstock your pond with fish as their excrement may be resulting in excess nutrients.  Some experts even recommend reducing fish food during an algal bloom to force the fish to eat algae instead of their regular food.

Check the run-off from gardens, lawns and even aquatic plants to make sure that fertilisers and chemicals aren’t ending up in your pond.


Increasing coverage of the pond’s surface area with floating plants and water lilies will decrease the amount of sunlight available to algae.


Algae will grow rapidly in a small, shallow pond where the water heats up quickly.  Again, lots of lilies and aquatic plants will help shade the water and keep the temperature down. However, you will notice that algae is generally at its worst in spring when water temperatures are on the rise, the sun is warmer and the plants haven’t started their new season growth.


Excessive surface vegetation can also result in too little oxygen in the water however, and this is detrimental to the growth of algae-eaters such as fish, aquatic snails and tadpoles. As you can see, keeping a pond healthy is all about balancing the ecosystem and keeping algae to a minimum does require an understanding hand.

But there are some types of algae that don’t need to be treated with any compassion – such as floating and string algae – which are also generally more of a problem during seasonal changes.  Getting rid of string algae requires some manual labour, and is best removed by twirling it around a stick or special pond wand.

There are also many handy pond supplies on the market which are very helpful in keeping algae to a minimum.  For example, an aerator will oxygenate the water whilst a vacuum can remove organic waste or sludge from the bottom of the pond, thereby preventing algae.

Minimising Algae in your pond

Having some algae in your pond or water feature is a good thing – but having pea soup clearly isn’t.  Knowing what causes algal growth and how to minimise it will go a long way towards making the life of a pond owner stress-free.

Remember, like any plant, algae needs food and light in order to photosynthesise and grow and therefore to control it, a number of factors need to be considered. Excess algae is generally more of a problem in new ponds because the ecological balance between plant and animal life hasn’t yet been established and there are no natural predators which would normally keep algal growth under control.

It goes without saying that regular maintenance, monitoring and management of your pond is important for algae control but let’s look at the most common causes of algae problems and what can be done to minimise them.

High levels of light

Abundant sunlight accelerates the growth of algae, so when designing and building a pond, the long-term aim should be to starve algae of light.  A dark colour on the pond’s interior is also important.  Other things that reduce sunlight (and therefore reduce the ability of algae to thrive) include shade from nearby trees, shrubs and walls and of course, cover from aquatic plants.

High water temperatures

Again, good pond design (steep sides, deep water) will mean the larger volume of water will take longer to warm up, thereby slowing down the growth of algae.   In other words, you need to maximise the pond’s volume relative to its surface area and remember, excess algae is more likely to be worse in the warmer months.

Pollutants in the pond water

Excess algae could also be caused by an excess of nutrients in the water.  Check that run-off from your garden or lawn or even from rain which could contain soil particles, fertiliser or chemicals isn’t going into your pond and feeding the algae.  It’s important to regularly remove excess leaves and decomposing organic debris from your pond in order to take away this source of nutrients for the algae.

Plant/fish imbalance

Having the right types and amounts of plants in the pond is important, as these absorb nutrients for themselves and therefore starve the algae of its food source.  A pond should have around two thirds of its surface covered by plants (e.g. water lilies) in order to reduce sunlight and lower the temperature of the water.   Algae also feeds on fish waste and fish food, so it’s important not to overstock your pond.

Poor water circulation

Installing the right pond equipment is vital in order to minimise algae growth.   Algae loves stagnant conditions, so it is important to keep the water moving with pond supplies like pumps and filters.

Chemical treatments can also be used to minimise algae, and the first step should be testing your water to identify deficiencies (e.g. wrong pH levels) and then getting the appropriate product to fix it. However, there are many different products on the market and it is important to discuss your needs with experts who specialise in creating, managing and enhancing aquatic environments.

Water treatment products are also very helpful, to help control all biological problems from algae to harmful sludge. Algae is a reality, but there are many things that pond owners can do to create a stable pond environment and reduce the prevalence of outbreaks.

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