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How to Control Algae in your Small Pond

There are hundreds of types of pond algae. The two most common are string algae which look like long green hair, and planktonic algae, which looks like green pea soup.  Many pond owners get very frustrated when trying to control algae.  I often get asked, “How can I kill the algae in my pond?”  This is the wrong question.  They should be asking, “How can I control the algae in my small pond?”  For this question, I have an answer.

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A Common Misconception

A common misconception is that algae are bad for a pond.  Only during a very out of control and heavy bloom is algae bad for a pond or fish.  This is rare in a small backyard pond.  Usually, the algae are irritating to the pond owner but not necessarily bad for the pond.  All-natural ponds will have some algae.  This is a sign of a healthy pond.  We need to control the algae by creating balance in our ponds.

Types of Pond Algae

String algae or filamentous algae will form mats that float on the pond’s surface, resembling wet wool. It will also grow along the pond’s bottom or edges where there isn’t much water circulation. Many pond owners refer to this kind of algae as “pond scum.”

Planktonic algae are floating microscopic plants that are usually suspended in the first couple feet of water, making the water look green and “pea soupy.”  This is the stuff that might prevent you from seeing your fish.

How to Control Algae

Before you can determine how to control algae, you need to understand what makes it grow.

Pond algae need two things to grow: nutrients and sunlight. By controlling these two things, we can maintain our pond’s balance, which will control the algae.  Sounds simple enough, right?


You can block the sunlight from the pond by using an aquatic sun blocker or pond dye, but this is unnatural and looks ridiculous.  It is much better and cheaper long term to plant a few water lilies in your pond to shade the water. 

Prevent Nutrients from Entering

You can prevent nutrients from entering your small pond by keeping leaves, debris, and runoff out of your pond.  Some people go overboard with this.  While it’s great to keep excess debris out of your pond, I’m guessing that you don’t have a bunch of time for this activity.  Some people are out skimming leaves from their pond daily.  I never do this. 

A well-balanced pond will be able to deal with a few leaves; what messes up pond balance is fertilizer runoff.  The worst algae blooms that I have seen were the result of runoff carrying lawn fertilizer into the pond or lawn fertilizer getting into the pond during application.

Chemical lawn fertilizers are very concentrated, and they can easily throw a pond out of balance.  Another very common way to accidentally fertilize your pond is by blowing grass clippings into it.  If you pay someone to fertilize and mow your lawn, take some time to watch them as they work.  Make sure that they aren’t adding to your pond nutrients.

Remove the Existing Nutrients

All ponds have some debris in them.  Adding beneficial bacteria to our ponds will help break down any debris that is already in the pond.  By maintaining high levels of bacteria, we can ensure that we don’t get any debris build-up in the pond.

Add Oxygen

Low oxygen levels in our water will slow down the decomposition of the debris.  High oxygen levels speed up decomposition and allow more beneficial bacteria to survive and help us out.  You can raise the oxygen levels in your water by installing a waterfall, a fountain, or a bottom aeration system.

Add Plants

Plants are our biggest pond helpers.  The pretty plants in our ponds are competing with the algae for the nutrients in our pond, so the more plants we have in the pond, the fewer nutrients will be available for algae.  Floating plants, submerged plants, and marginal aquatic plants can all do their part to help balance our ponds.

Most garden ponds do not have enough plants in them to handle all the nutrients in the water.  Planting more plants will help reduce algae, But what if we could install something to supercharge the removal of nutrients?

Install a Wetland Filter

Upflow wetland filters mimic the natural wetlands that filter all of the earth’s water.  By installing a wetland filter in your pond system, you provide your pond with the absolute best filtration available.  A wetland filter can be planted with many aquatic plants, and these aquatic plants that are in the wetland filter with pond water getting pushed through their roots will grow more quickly and remove more nutrients than plants that are growing in the pond.

Install a Plant Island

Plant islands float in your pond and give you even more space to plant.  They also allow the plant roots to hang down into the pond water and soak up nutrients just like nature bog plants do.  These plants will also take in more nutrients than the pond bottom plants.

Add Barley

When barley straw decomposes in water, it releases a natural toxin that slows and discourages algae growth.  Put a small bale of barley straw in your small pond, and you will help control the algae cheaply and naturally.  The barley should be placed somewhere near the surface or below a waterfall in the pond.  Once again, more oxygen will help the barley decompose more quickly.  Moving water is our friend.

Add Fish

Many people think fish are an option when you own a pond.  I consider fish to be a vital part of pond balance.  Your fishy friends will spend their days rooting around through the gravel on the bottom of your pond, stirring things up and eating the algae.  Be careful not to feed your fish too often; hungry fish make for a clean pond.  Any fish food you toss in the pond that doesn’t get eaten becomes fertilizer for algae.  

Never, Ever Add Chemicals

Chemical products designed to kill algae are never a good idea.  Killing algae is not the answer; controlling it naturally is.  Any chemical killers that get into your pond water kill off beneficial bacteria and beneficial microscopic pond dwellers that help keep our pond balanced.

Killing the algae results in dead algae, dead bacteria, and dead pond creatures piling up at the bottom of the pond, just waiting to be fertilizer for the next algae bloom as soon as the chemicals wear off.  It is a never-ending vicious circle, don’t do it.

Wrapping it Up

If we understand that our small backyard ponds are natural ecosystems and that they will take care of themselves if they are correctly balanced, we can understand how to control our algae.

If we have heavy algae blooms, we have too many nutrients in our pond.  Killing algae will only make it worse.  We need to eliminate the excess nutrients, not the algae.

We can do this by keeping the nutrients out of the water or by removing nutrients from the water.

Keep debris and runoff out and add plants, bacteria, and fish to promote balance.  Balance will take a bit of time and patience to achieve, but everything gets much easier once we achieve this balance.

Working with nature is always the easy way.  Working against nature will only bring us trouble.

Keep it simple and be a part of the solution, not the problem!