How to Kill Blanketweed

Updated April 17, 2017

Blanketweed, otherwise known as string algae or simply pond scum, is a green algae that can wreak havoc in any koi pond, especially in hotter months. This is caused when sunlight reacts with an excessive amount of nitrates and phosphates in your water, typically from your koi or goldfish waste. This slimy coating of stringy fibres can make even the most seasoned aquarist throw up her hands in defeat. But with the right preventive measures and a quick response, you can have your pond blanketweed-free in a week or so.

Use a skimmer net to remove as much of the blanketweed as you can.

Install the proper filtration equipment appropriate for your pond. Consult the manufacturer to make sure that your equipment is suitable for your size of pond. Without proper filtration, nitrate and phosphate levels will become unhealthy for your fish.

Add a pond treatment that's advertised to reduce nitrates and phosphates in your pond. Use the amount recommended by the manufacturer.

Add barley straw to your pond, available from area farmers. This is a natural algicide. Other commercial algicides may be more effective, but barley straw extract is often the active ingredient. Several choices are available online.

Shade your pond from sunlight. Plant shade trees near your pond to reduce the amount of sunlight that reaches your pond. Construct a pergola over a large portion of the pond, if possible. Conversely, you can locate any new ponds in already-shaded areas.

Introduce shading plants to your pond that will shade the water underneath, such as lily pads. Add vegetable dyes to your pond water, which also filter out sunlight and give your pond an aesthetically pleasing tint. These are available at many landscaping supply stores.


Regularly inspect your pond for regrowth of blanketweed and remove it with your skimmer net as soon as it's spotted. Completely removing this underwater weed will take persistence.


Many chemical algicide treatments, such as those containing copper, have been banned in several countries for being damaging to fish and plant life. Make sure your algicide isn't one of them.

Things You'll Need

  • Filtration equipment
  • Koi pond treatment
  • Skimmer net
  • Shading aquatic plants
  • Shade tree or pergola
  • Vegetable dye
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About the Author

Dan Harkins has been a full-time journalist since 1997. Prior to working in the alternative press, he served as a staff writer and editor for daily publications such as the "St. Petersburg Times" and "Elyria Chronicle-Telegram." Harkins holds a Bachelor of Arts in journalism from the University of South Florida.