Why Is There Lime Green Algae in My Pond?


If lime green algae begins to cover the surface of your pond, it most likely indicates your pond has a healthy crop of filamentous algae. Also called blanket weed, filamentous algae is common in ponds but can leech nutrients away from your aquatic plants.

The Facts

Filamentous algae in your pond will prevent more dangerous varieties of algae, such as blue-green algae or planktonic algae. Filamentous algae is relatively simply to remove with a stick or garden tool as it clumps together tightly on the water's surface.


Filamentous algae will float on the surface of the water and also grow around rocks and water features. It is typically bright green but may be yellow or even brown. Filamentous algae may appear in long, thin strands or resemble a thick, green carpet on the surface of your pond.


Some forms of pond life may feed off the filamentous algae that floats on the surface of the water. The thick mat of algae also provides an ideal cover for animals such as snails and small fish.


To reduce the levels of filamentous algae in your pond, use EPA-registered sodium carbonate proxy hydrate products to kill the algae, or introduce grass carp into your pond. Grass carp feed on filamentous algae.


When the algae begins to die and decompose, it releases harmful chemicals into the water, and the decomposing plant matter rapidly depletes the pond's available oxygen, which fish and plants need to survive.


  • Minnesota Department of Natural Resources
  • State of Washington Department of Ecology

Who Can Help

  • Otocinclus.com: Types of Algae and How to Control Them
  • Naturalenviro.com: Filamentous Algae
Keywords: filamentous algae, pond, surface of the water

About this Author

Ciele Edwards has been writing for more than 10 years. She has authored a credit repair guide that is currently in use by The Credit Repair Network and its business affiliates. Edwards holds a Bachelor of Arts in English and is currently a technical writer and credit specialist.