Why is there string algae growing in my pond?

String algae is a normal and healthy part of having an ecosystem pond. This single celled aquatic plant comes in a variety of species but will commonly be green and grow in long strands. Algae can be quite beneficial to your aquatic ecosystem since it utilizes excess nutrients and waste, produces oxygen, and acts as a food source for certain fish and macroinvertebrates.

 However, it is often unsightly to us as pond owners since we expect to have clear, clean water that allows us to see all our fish, rocks, and plants. Too much algae growth can also cause a crash in the dissolved oxygen levels of your pond if it all dies off quickly. So what causes excess growth of string algae and what are the solutions?

The number one cause of string algae growth is an excess amount of nutrients in the pond, usually phosphorus. These nutrients can come from a variety of sources such as fish waste, decaying organic debris, eroding soil/mulch, etc. Make sure your pond’s water volume and filtration is sized appropriately for the amount of fish you have. A good rule of thumb is 1 inch of fish per 10 gallons of water. Check out our information about wetland filters in case your filtration needs to be upgraded. Overfeeding your fish can also yield too many nutrients in the pond so try cutting back feedings if algae blooms are persistent.

A build up of organic debris in your pond such as leaves, dead foliage, and mulch can add a lot of nutrients as they decompose. Algae will flourish in ponds that don’t have skimmers or get fully cleaned regularly. Check out our cleaning services if it’s been awhile since your pond was last cleaned. Installing a leaf net in autumn to keep out the falling leaves will also help to alleviate this problem.

If your pond is in full sun, even if there is very little nutrients in the water string algae can still proliferate. Planting trees, putting up a sun shade sail, or growing more water lilies will all help to shade the pond and limit algae growth. Since well and city water can also be sources of nutrients, consider adding a rainwater harvesting system to your pond to constantly bring in fresh, clean water.

After you make sure there is no soil washing in over the liner, the next step in preventing algae growth would be to regularly add beneficial bacteria and introduce more aquatic plants. Establishing strong bacteria populations in the filter and gravel will help outcompete the algae for nutrients along with the other plants. Since these bacteria populations will naturally fluctuate it’s important to regularly add bacteria treatments to boost their numbers. 

String algae should be physically removed as much as possible before algaecides and other treatments are used. There are a few good options for treating stubborn string algae problems. The most effective treatment solution that we’ve used is the IonGen System. The Iogen releases copper ions into the water which target since celled algae but are safe for other plants and fish. Learn more about the Iogen here or contact us if you’d like one installed. ​

Iongen System

Aquascape Ecoblast is a great option to treat algae growing in the waterfall or stream. It oxidizes and kills algae on contact so the pump should be turned off, Ecoblast sprinkled on the algae and allowed to work for at least 30 min before turning the pump back on. Ecoblast does not work well at treating algae that is underwater.
Aquascape Algaecide is better suited for targeting algae growing in deeper areas of the pond. Make sure to read the label and apply the proper dose for the amount of gallons in your water feature. We also advise applying the algaecide right after the biofalls to allow the treatment to be more diluted before it reaches the beneficial bacteria growing in the biological filter.

Before Algaecide

After Algaecide

Sting algae can be a headache to deal with but following these steps to achieve a properly balanced and maintained aquatic ecosystem will keep algae growth to a minimum.