A Comprehensive Guide On How To Remove String Algae From Your Pond

Algae, an unsightly, green menace, often becomes a common adversary for koi keepers and water garden enthusiasts, especially during warmer months. The occurrence of algae can diminish the aesthetic appeal of your pond, obstruct visibility of your koi, and even hamper the functionality of pond equipment. Herein, this article aims to provide detailed insights on how to deal with one specific form of algae - the stubborn string algae or blanket weed. It includes both preventative measures and remedies to keep your pond algae-free.

A Seasonal Issue of Algae in Koi Ponds & Watergardens

The battle with algae for most koi keepers begins in early spring, with rising pond temperatures acting as the catalyst. The increase in temperature stimulates nutrient proliferation necessary for algae growth, thus leading to its proliferation. Conversely, winter months usually result in a diminished algae problem due to reduced nutrient content resulting from the pond's slowed-down dynamics, less active koi, and more dormant filters.

Identifying the Algae Culprits

Among the thousands of algae species globally, two main types frequently invade our koi ponds - planktonic algae and string algae.

Planktonic Algae

Resembling ‘pea soup’, planktonic algae make their appearance either in a newly installed pond as the water starts maturing or if the pond doesn't have a UV sterilizer. However, it isn't usually a significant problem as a properly-sized UV sterilizer can eliminate it quite efficiently. Once the filter has dealt with the smaller algae particles, clear water is restored.

String Algae

String algae, commonly referred to as blanket weed, present a more challenging issue. It can quickly take over your pond, disrupting the filter's functionality and complicating pond maintenance. UV filters cannot prevent this type of algae because it doesn't pass through the pond pump. It's more likely to obstruct your pump and limit your water flow - a scenario one should avoid.


The unfortunate news is that completely eliminating string algae from your pond might not be feasible. Some ponds may never encounter it, but others might find it returning year after year. However, this should not cause undue worry, as there are measures to prevent and control string algae infestation.

String Algae Prevention: Proactive Steps

The adage "prevention is better than cure" holds especially true when managing string algae. Here are expert-recommended preventative steps:

Detritus Removal

Remove any detritus, including dead or rotten leaves, from the pond before winter arrives. Consider covering your pond before the autumn leaf fall to prevent further contamination.

Sunlight Restriction

Since algae require photosynthesis to grow, limiting sun exposure can curtail blanket weed growth. Using a shade or pergola can restrict sunlight while maintaining the pond's aesthetic appeal.

Regular Nitrate Testing

Monitor your pond's nitrate level regularly. High nitrate levels provide a fertile ground for algae growth.

Nitrate Level Reduction

Regular water changes can help reduce nitrates, providing fewer nutrients for algae to thrive. Always test the Nitrate levels in your source water to ensure you're not inadvertently adding to the problem.

Controlled Feeding

Avoid overfeeding your koi. Ensure no food is left over after feeding times. If you find leftovers, reduce the quantity for the next feeding.


During the summer months, use lots of aeration. It benefits the fish and prevents the water from becoming stagnant.

Adding Plants

Pond plants can significantly contribute to algae control. They outcompete algae for nitrates and nutrients and provide shade, reducing sunlight exposure. However, ensure dead or dying plant debris doesn't accumulate in the pond, as it could encourage future algae growth.

String Algae Removal: Reactive Measures

Despite best efforts, string algae might still appear in your pond. In such cases, some reactive measures can help:

Manual Removal

  • Manual removal using a garden rake, pond net, or even a shop vac may become necessary if the algae have taken over your pond or clogged your pumps. It might be a messy affair, but sometimes it's the best approach.

Chemical Removal

  • Chemical treatment is a last resort and comes with a strong word of caution. Overdosing or overusing algaecides can quickly decimate your entire koi collection. If you must use chemicals, monitor your pond closely during treatment and be prepared for a large water change if necessary. Know your exact pond gallons and dose accordingly.

Conclusion: Striking the Right Balance

Algae, especially in small amounts, are a natural part of your pond's balance. Depending on the season, algae can appear at varying times throughout the year, with a typical surge from spring to autumn, followed by a winter die-back. With a little foresight and prevention, you can keep your pond's algae levels under control.

Remember, dealing with algae isn't an insurmountable task. It involves learning about your pond, its inhabitants, and how best to maintain this delicate ecosystem. Feel free to ask any further questions or share your experiences about algae prevention or treatment in the comments section below.