How To Use Hydrogen Peroxide For Pond Algae

Hydrogen peroxide is a common homemade DIY treatment for algae overgrowth in backyard ponds.

If you’re struggling with algae, particularly string algae on rocks or waterfalls, then hydrogen peroxide could be an effective natural solution.

In this article I’ll tell you everything you need to know about using hydrogen peroxide to clear your pond water.

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pond aeration

The reason hydrogen peroxide works well as a natural pond water cleaner is because it has anti-bacterial and anti-fungal properties.

Hydrogen peroxide helps to remove algae quickly, while also increasing the oxygen level of the pond water.

What is hydrogen peroxide?

Hydrogen peroxide is a colorless natural liquid similar to water.

It’s made up of two molecules of hydrogen and two molecules of oxygen, with a chemical symbol of H2O2. [1]

Hydrogen peroxide is commonly used as a mild antiseptic for cuts and you probably have some in your bathroom cabinet or first aid kit.

Many pond owners use hydrogen peroxide to clear up algae overgrowth in pond water because it’s a natural treatment that won’t harm fish.

What causes algae overgrowth in ponds?

Algae blooms can be caused by:

  • Fish overcrowding
  • Stagnant water with no aeration
  • Decaying plant matter
  • Not enough pond plants

Algae overgrowth not only looks bad, it also decreases the amount of oxygen in the water which can cause fish to die, especially during periods of hot weather.

pond with green water

How to use hydrogen peroxide to remove algae

First, measure the width, length and depth of your pond to determine the volume.

Use 1/2 cup of 3% hydrogen peroxide per 100 gallons (378 litres) of pond water.

Pour the peroxide into the pond away from the fish and stir the water around to mix it in.

Hydrogen peroxide can damage the gills of fish, so take care not to pour it directly onto the fish.

Don’t use more than the prescribed amount because it can cause the algae to die off too quickly, leading to an ammonia spike which is dangerous for fish.

The peroxide will take effect within a few minutes, and the increased oxygen will last for a few hours.

Hydrogen peroxide can cause the sediment from the bottom of the pond to come up to the surface, so be ready to scoop it out with a fine net.

Another way to get the benefits of hydrogen peroxide without using the liquid is to add barley straw to your pond.

As the barley straw breaks down it naturally produces hydrogen peroxide, which helps to control algae and reduce any nasty odors coming from the pond.

pond water

Will hydrogen peroxide clear up green pond water?

If your pond water is looking like a thick green soup, it’s too far gone for hydrogen peroxide to help.

In this case, you’re best to use a UV clarifier to destroy the algae and remove it from the water.

You can read more about clearing up green pond water in this article:

How To Fix Green Pond Water

Is hydrogen peroxide safe for pond fish?

Yes, hydrogen peroxide is safe for fish as long as you follow the instructions and pour it into the pond away from the fish.

Hydrogen peroxide is only a temporary solution and continual use can damage the koi’s gills.

Safety considerations

Be sure to keep hydrogen peroxide away from small children and pets.

Avoid treating the algae in very hot weather because the die off can dramatically reduce the oxygen level in the water.

So there are my tips for using hydrogen peroxide to clear pond water.

Hydrogen peroxide is particularly helpful for removing string algae on rocks and waterfalls but it’s important to remember that hydrogen peroxide is only a temporary measure to remove algae.

If you’re constantly battling with algae overgrowth you’ll need to address the root cause which could be fish overcrowding, decaying plant matter, too much sunlight, increased nitrate and phosphate levels, stagnant water with no filtration or a lack of pond plants.

Here’s a quick video that shows how to treat pond algae with hydrogen peroxide.


Have you tried using hydrogen peroxide or any other homemade DIY algae treatments in your pond? Let me know in the comments below.

Are you on Pinterest? I have boards dedicated to Backyard Ponds and Pond Tips that you may find interesting.

hydrogen peroxide for pond algae

Also, check out the Must-Have Pond Products page where I list the essential pond products to make life as a pond owner much easier!

Kelly Martin

Hi, I'm Kelly Martin, a landscape gardener with a special interest in ponds and water gardens. For over a decade, I've been designing and creating beautiful gardens that feature stunning water features, from small garden ponds to large water gardens. Read more

This Post Has 2 Comments

  1. Judith

    How often do you use peroxide

  2. Aprill

    I love this article. Hydrogen peroxide can be safely used in ponds, aquariums and live wells. Yes, we must use caution and follow suggested measurements. I have saved many fish from dying by using hydrogen peroxide. Some people give poor advice about the benefits of hydrogen peroxide. Thank you for this excellent article.

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