Filamentous Algae

Commonly called string or hair algae, filamentous algae typically clings to the sides of  ponds, pond plants, pumps and waterfalls. Typically it grows a few inches long, but if left unchecked in moving water, it can grow up to several feet long. The very presence of string algae is an indicator that the water conditions are suitable for aquatic life.

Benefits of String Algae

  • Acts as a natural filter to control ammonia and ammonium, which can be toxic to aquatic life. It absorbs ammonia/ammonium and nitrates from the water as a source of nutrition.
  • Provides a rich food source for pond fish .


  • Creates a soft spawning site for fish.
  • Provides necessary oxygen to ponds during the day and uses oxygen at night.


Balance and Moderation are Key

Like many things, string algae can be beneficial to your pond in small amounts. But like cookies or cake you can have too much of a good thing. When hair algae grows unchecked it can cause various issues in your pond. Besides being aesthetically displeasing it can clog pumps and filters, choke out other wanted plants and cause pH fluctuations.

The very first thing to remember is that algae only needs two things to survive, food and sunlight. When it gets too much of either, it becomes a problem. When it gets too much of both it can quickly spread across a pond. To help curb the growth of algae you can create a more balanced pond environment.

Here are our tips on how to create a balanced pond:

  • Add lots of oxygenating plants which absorb excess nutrients. The more plants that feed off these excess nutrients the less algae has to eat, and algae will eventually starve. We recommend using Hornwort.
  • Make sure you have adequate circulation. Proper circulation of your pond means that the total volume of your pond should be circulating at a rate of once per hour.Example: if your pond is 800 gallons you would want a pump that pushes the water at 800 gallons per hour.
  • Include a fountain, waterfall or aerator to provide oxygen to aquatic life. Proper levels of oxygen also helps in the maintenance of beneficial bacteria and the breakdown of waste.
  • Add beneficial bacteria to your pond every week. Healthy bacteria culture in a pond creates competition for algae. We recommend Bacta-Pur products.
  • You should have 60-75% of your pond surface covered by plants. String algae thrives in sun, the more sun the more shade you need to create. We recommend using water hyacinths, water lilies and duckweed. You can also use pond tints which filter out the UV rays and create shade. We carry them in 3 different colors to choose from.
  • Remove any heavy debris as best as you can. By letting leaves or maple keys decompose in the pond you are creating food for algae to feed off.
  • Do you have an abundant amount of fish? You may want to consider reducing your fish population or cut back on frequent feedings. Fish waste not only creates food for algae it also releases ammonia into your pond.
  • Add barley straw which slowly breaks down in the pond and works as a natural algaecide.
  • Don’t forget to treat your pond with pond conditioner any time you add water! When we add water to our ponds we also add chlorine and chloramines. These two by-products of our water sanitation programs are not healthy for aquatic life and need to be eliminated. We recommend using Pond Detoxifier in order to remove heavy metals and chlorine.


Taming the Beast

While a balanced pond is an essential ingredient in taming string algae, in some cases it simply is not enough. This is particularly the case in ponds that feature waterfalls and streams, as they provide a great surface for algae to grow. By their nature, waterfalls and streams are unlikely to be shaded and provide string algae with all the light it needs to prosper. Once the algae gets a foothold in these areas it can spread throughout the pond.

Here are our tips on taming the growth of string algae:

  • With many things, a little bit of elbow grease is the best remedy. Try and physically remove as much of the excess string algae as you can.
  • Growth on waterfalls can be significantly reduced by periodically scrubbing the rocks with non-iodized sea salt. Allow the rocks to dry thoroughly before turning your waterfall back on.
  • If you don’t have fish you should consider adding a couple. Fish live on a mixed diet that includes a substantial amount  of vegetative matter like string algae.
  • Add tadpoles and snails as they both rely almost entirely on string algae as a food source.
  • Consider adding a pond magnet to your pond’s circulation system. Passing water through a magnetic field changes the size, shape and composition of mineral salts in the pond. It has been found that these fluctuations reduce the ability of string algae to absorb nutrients resulting in elimination of the algae.
  • For immediate control of string algae on waterfalls or streams you can use EcoBlast which breaks down string algae on contact.
  • We also recommend using Pond Debris and Clarifier. We have had really good feedback from customers on this product with the control of hair algae and murky water.

If you still have an issue with the growth of string algae, bring in a water sample for us to test. Sometimes the pH levels in your pond can be out of whack and promote the growth of string algae.

Do not confuse string algae with super beneficial slime algae (the slippery feeling on pond liners and rocks). Slime algae produces 70% of the oxygen in the pond and creates an ideal surface to grow beneficial bacteria.