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The Raymar Blog

How to Control Pond Algae… Naturally!

Posted on July 13, 2022 
by Thomas VanAmerongen

Every spring, we get frantic calls from new pond owners who are worried about large amounts of algae floating in their pond and wondering what to do about it.

So what exactly is algae… and will it harm your pond and the fish who call it home?

Here’s everything you need to know about algae (without getting too scientific) so you can see it for what it is, and know what to do when it gets out of control. Let’s go!

Pond algae on a garden feature.

What is algae?

Algae is common in ponds and is a normal part of a healthy ecosystem, providing food for fish and other aquatic animals (and 70–80% of the oxygen we breathe!). Although some consider it an aquatic plant (because it uses photosynthesis to create oxygen) algae is actually a protist — a member of a diverse collection of organisms that don’t fit into animal, plant, bacteria or fungi groups.

Algae is also an autotroph, meaning it produces its own food using sunlight, water, carbon dioxide and available nutrients, like phosphorus and nitrogen.

So while algae is part of a healthy ecosystem, too much of a good thing can lead to “algae bloom.”

Algae bloom in a pond.

What is algae bloom?

When algae becomes excessive it creates what is known as “algae bloom” — a condition that releases harmful toxins into the water and suffocates your fish by depleting the amount of oxygen available in the water.

Algae bloom occurs when there are too many nutrients in the water, specifically nitrogen and phosphorus.

It’s quite common in the springtime, because the surface water warms up before the water closer to the bottom, leaving it too cool to support beneficial bacteria that help “eat” the excess nutrients. And aquatic and marginal plants are not actively growing in early spring, further reducing the competition for nutrients.

Natural pond.

How can you prevent algae bloom?

Although unsightly, it’s not usually a cause for concern, as it usually clears up on its own once the water warms up (and the beneficial bacteria and water plants get to work).

But if you want to reduce the amount of bloom in the spring, aerate the water once the ice has melted.

The benefits of aeration to reduce algae bloom are two-fold: by creating a more uniform temperature, the water at the surface stays cooler, inhibiting algae growth. And by warming up the water at the bottom, beneficial bacteria can begin processing nutrients much sooner.

There are other ways to prevent/reduce algae bloom in your pond:

  • Don’t allow lawn and garden fertilizer to leach into your pond’s water — The chemicals used in fertilizers are nitrogen, phosphorus, and potassium… perfect food for hungry algae! Keep fertilizer at least ten feet away from your pond, and use a fertilizer that is low in phosphorus (the second number on the bag indicates the amount of phosphorus. The smaller the number, the safer it is for your pond).
  • Divert stormwater and runoff away from your pond — If runoff is an issue, try digging a trench to divert the flow of water away from your pond (runoff is one of the reasons we don’t recommend locating your pond in a low, wet area of your property).
  • Avoid overcrowding your pond with fish — Fish produce waste, so having too many will raise the nutrient levels in your pond. The general rule of thumb is to have no more than one fish for every hundred gallons of water (average length x average width x average depth x 7.48 will give you the total number of gallons in your pond).
  • Don’t overfeed your fish — Overfed fish can develop health issues (like fatty liver and fin rot) and create an environment high in nitrates and ammonia. This is toxic to fish, but algae love it! When feeding your fish, remove any food that isn’t eaten in five minutes.
  • Surround your pond with aquatic and marginal plants — Including a variety of plants in your pond design is a beautiful way to help reduce the amount of algae in your pond (see our post Choosing the Best Plants for Your Pond for ideas). The plants compete with the algae for the nutrients and block the sun, making it difficult for algae to grow and multiply. Aim to cover as much of the water’s surface with aquatic plants as possible.
  • Provide proper filtration for your pond — There are four types of pond filtration that keep your pond clear and healthy — mechanical, biological, chemical and ultraviolet clarification (we go into these in depth in How to Keep Your Pond Clean and Healthy).
Aerate water in a pond.

How can algae be controlled without harming the environment?

There are two basic types of algae that appear in most backyard ponds — suspended algae and sting algae.

Both of these can be controlled following the prevention methods listed above. If you want to get rid of algae quickly (without resorting to chemicals) you can physically remove the algae using a skimmer or rake (you can use scissors to snip the string algae).

Bear in mind that simply removing the algae won’t fix the problem… you still have to deal with the issues that caused the algae to take over in the first place.

You can also throw a bale of barley straw into your pond to held rid your pond of excessive algae. As the barley straw rots it releases hydrogen peroxide, which kills off algae in a matter of weeks without harming fish or other plant materials. Use eight ounces of barley straw for every 1,000 gallons of water.

Control pond algae with surrounding rocks.

Need help caring for your pond?

Caring for your pond can be time-consuming, especially if you’re a new pond owner.

If you want to leave your pond’s maintenance to the professionals — leaving you plenty of time to relax and enjoy your pond — get in touch. We have three different service plans, so you can choose the one that’s best for you.