Skip to main content

Pond Algae - Advice and Treatment

Published on July 27th 2019

by PimlicoDan. All rights reserved

A close up of an animal in the water
I remember getting my first pond for my tenth birthday. I was so excited as I watched it fill with crystal clear water - I could see the pondweed, the fish darting about. It all looked very blissful for about two weeks until the water turned pea soup green.
It then became overrun with hair algae, and I became sad, thinking I had done something wrong. Sound familiar? Unfortunately, for many people, the initial delight of a garden pond is outweighed by the problems that ensue.
To try and save you pain and heartache, here are a few tips on how to prevent and treat it algae, in its many guises.
A white and green broccoli
A close-up of algae, showing each individual cell. Much of the suspended algae which causes water to turn green is single-celled.

What is algae?

Algae are primitive, tiny plants. They photosynthesise using light, water, CO2 and the nutrients available. However, they have a staggering growth rate, which is why the appearance of algae can be such a shock to new water gardeners.
Although algae produce oxygen by day, at night it respires, taking in oxygen, sometimes depleting it to dangerously low levels.
N.B. Although algae are the plural of alga, we tend to use it singularly when discussing it as an issue.
Algae will often appear as green water (main image), blanketweed/floating scum and hair algae.
A close up of a piece of broccoli
The water gardener's nightmare. A close-up of blanketweed, showing its scum-like texture.

Why does it appear?

The reason new ponds suffer an algal explosion is down to a few reasons.
  • Chemicals and nutrients in tap water or a substrate.
  • No plants - or new plants have yet to establish.
  • Open water surface exposed to direct sunlight.
  • Lack of an established biological cycle, particularly if overstocked with fish.
Some of these things are difficult to avoid in the first instance, but you can make provisions in the long term.
A close up of a pond
Water lilies are the ideal solution to the problem of algae, but can take a season or two to settle in.

Preventing algae

Planting particular plants can solve many of the issues above. Oxygenators, marginals and floating plants soak up excess nutrients from the water before algae can use them and floaters will also screen the water’s surface.

White Water Lily

Nymphaea alba


Lemna spp.


Hydrocharis spp.

Direct sunlight and nutrients are like catnip for algae, so using plants like duckweed, frogbit and waterlilies will stop algae dead. Just be aware some floating plants are a pest in themselves (see article linked below).
Other plants, like bogbean, brooklime and bog arum, dangle their roots freely in water, again siphoning off excess nutrients and reducing surface area. Ideally, plants should cover one-third of the new pond’s surface.
A close up of a green plant
Brooklime (Veronica beccabunga) will stretch out into the water and suck up nutrients. Its leaves are occasionally used in salads.
Download the free Candide App to get help and answers from a warm community of gardeners
Download on the App StoreGet it on Google Play
Careful stocking:
A big problem with new ponds is overstocking with too many fish. A pond takes a while to establish a biological cycle, where good bacteria break down harmful chemicals such as ammonia. Fish waste and overfeeding, coupled with a lack of an established cycle, means that algae can thrive on the excess nutrients.
The solution here is to introduce fish over time, not all on the first day. This is also good for the fish as they'll find it infinitely less stressful with fewer chemicals in the water and more places to hide in established vegetation.
Although some fish will eat certain types of algae, they are always more likely to be a cause rather than a cure for green water, particularly in small ponds. So be mindful of how many fish your pond can sensibly hold.
Food on the water
Fish produce a considerable amount of waste and can eat beneficial water creatures which may feed on suspended algae. So careful feeding and stocking is essential.
Outside influences:
If you can locate the pond in light shade, all the better. Also, ensure it has a reasonable depth (the usual advice is over 60cm/2ft) to avoid overeating in the midsummer sun.
In the autumn, leaves from nearby trees may fall into the pond. Not only will these release harmful chemicals, but they will also then rot to form sludge, a pre-made meal for algae come the springtime. Placing netting over your pond will prevent this and although you can remove sludge, be aware that many amphibians use this to hibernate in over winter.
Lastly, be conscious of any plant food leaching into the pond from the surrounding garden.
A man that is standing in the grass
A large-scale clean-up is sometimes the only way to control a bad case of blanketweed.

Treating algae

Even following the rules above, you may still struggle, and once algae gets a hold, you may need to look into treatments.
Manually: Hand removal is the easiest solution for blanketweed, and you can use a rake on larger ponds. It will grow back quickly, but if you can make a few changes with planting, you may be able to resolve it. You can scrub or blast hair algae with a hose, but be careful not to damage the pond lining or established plants/fish.
Barley straw: This has been used for years in the control of algae. Nobody's entirely sure how it works- some people say it doesn’t work at all but is merely an old wives’ tale. You will still need to remove blanket weed in advance as barley straw is more of a prevention than cure.
Chemicals: There are several products on the market for the treatment of algae, particularly for green water. They can be pricey and should be used as a last resort. Unfortunately, they usually need repeated treatments as they don’t solve the problem, merely remove the symptom.
Pumps, filters and clarifiers: The movement of water around the pond will help to keep it oxygenated, but they aren’t a solution. There are water clarifiers on the market, which use UV (ultraviolet) light to kill suspended algae and then clump it together, ready to be filtered out. This is the most expensive method of treatment. However, it doesn’t rely on chemicals so could be used in a broader variety of settings.
A group of orange flowers in a garden
A healthy, well balanced pond takes time and patience, so don't lose hope!

Related articles


Floating Plants for the Water Garden

Surface-dwelling plants offer some of the most beautiful and curious ornamentals around, from the fairytale waterlily to the...
A close up of a green plant

Slow reads


Why do plants need water?

Benjamin Franklin once famously said that there are only two certainties in life – death and taxes. He clearly wasn’t a...

Tucking in the Toads

Few garden visitors are as helpful as frogs and toads. They relish eating up our pests, even slugs. They are still visible in...

Love gardens? Sign up for Candide’s Almanac!

A weekly edit of freshly picked gardening tips, travel guides, and the best botanical days out happening near you. Unsubscribe at any time.



About usCareersPrivacy policy

Candide is your guide to visiting UK public gardens. Find the best gardens, buy tickets and enter with just your phone. Download the app for offline tickets, community access and more.

Terms & ConditionsCode of Conduct

© 2023 Candide

Made in Bristol