How to Get Rid of Brown Algae in Koi Pond

By Algal Web

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A koi pond is a beautiful addition to any garden or outdoor space. However, the presence of brown algae can quickly turn this tranquil oasis into a murky and unsightly mess.

In this article, we will explore the causes of brown algae growth in koi ponds, the potential risks it poses, and most importantly, how to effectively get rid of it. 

By following the preventive measures and removal techniques outlined here, you can restore your pond to its former glory and ensure a healthy environment for your beloved koi.

Understanding Brown Algae

Brown algae, also known as diatoms, are a common type of algae that often plague koi ponds. They are characterized by their brownish or yellowish appearance and slimy texture. Brown algae thrive in nutrient-rich water and are typically caused by imbalances in the pond ecosystem. 

Excessive sunlight exposure, high nutrient levels, and poor water quality can all contribute to their rapid growth. If left unchecked, brown algae can deplete oxygen levels, block sunlight from reaching aquatic plants, and even suffocate your koi.

Prevention Measures

Prevention is always better than cure, and when it comes to brown algae, it is crucial to maintain a well-balanced pond ecosystem. Here are some effective preventive measures:

  • Proper filtration system and regular maintenance: Install a reliable filtration system that can efficiently remove excess nutrients and organic matter from the water. Choose a system that includes mechanical, biological, and chemical filtration methods. Regularly clean and maintain the filters to ensure optimal performance.
  • Appropriate fish population density: Avoid overcrowding your pond with too many koi. Overstocking can lead to excessive waste production, which serves as a food source for brown algae. Maintain a proper balance between the pond size and the number of fish. Consult with a knowledgeable expert to determine the ideal fish population density for your specific pond size.
  • Balanced sunlight exposure and shade coverage: Excessive sunlight can trigger brown algae growth. Provide adequate shade using aquatic plants, lily pads, or artificial covers to limit sunlight penetration. Consider the orientation of your pond and strategically place shade-giving structures or trees to shield the water from intense sunlight.
  • Nutrient control and avoiding overfeeding: Monitor and control nutrient levels by reducing the amount of fish food and fertilizers used. Overfeeding leads to excess nutrients that fuel algae growth. Feed your koi only what they can consume within a few minutes. Remove any uneaten food to prevent decomposition and nutrient release into the water.

Removal Techniques

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If brown algae have already invaded your koi pond, prompt action is necessary to prevent further damage. Here are several effective methods for removing brown algae:

  • Manual removal using nets or brushes: Gently skim the surface of the pond water with a fine-mesh net to scoop out floating algae. For stubborn algae attached to pond surfaces, use a soft brush to scrub them away. Take care not to harm your koi or disturb the pond substrate. Repeat this process regularly to prevent algae from taking over again.
  • Chemical treatments, if necessary: Chemical treatments can be used as a last resort when other methods fail to control brown algae growth. Consult with a professional or knowledgeable retailer to choose an algaecide specifically designed for use in koi ponds. Follow the instructions carefully and consider temporarily relocating your koi to a separate holding tank during treatment. Note that chemical treatments should be used sparingly and as a temporary solution to avoid adverse effects on the pond ecosystem.
  • Introduction of algae-eating organisms: Introduce algae-eating organisms like snails, shrimps, or certain fish species (e.g., Siamese algae eaters, plecos) that naturally feed on brown algae. These organisms can help keep the algae population in check. However, exercise caution and ensure compatibility with your koi and the overall pond ecosystem. Some algae-eating species may have specific dietary requirements or may compete with koi for food, so research and consult with experts before introducing them.
  • Consideration of natural remedies or UV sterilizers: Some natural remedies, such as barley straw or beneficial bacteria additives, can help control brown algae growth. Barley straw releases compounds that inhibit algae growth when decomposing in water. Beneficial bacteria products enhance the biological filtration process, reducing excess nutrients. UV sterilizers can also be effective in controlling algae by exposing the water to ultraviolet light, which disrupts algae cells’ ability to reproduce. Install a UV sterilizer unit to the pond filtration system and ensure proper maintenance and replacement of the UV bulb.

Restoring Pond Balance

After successfully removing brown algae, it is crucial to restore a healthy pond ecosystem to prevent its recurrence. Follow these steps:

  • Regular water testing and necessary adjustments: Test the water regularly for parameters such as pH, ammonia, nitrite, nitrate, and phosphate levels. Make necessary adjustments using appropriate water treatments or additives to maintain optimal water quality. Conducting routine water tests helps detect any imbalances early on and allows for timely corrective measures.
  • Beneficial bacteria and enzymes for biological filtration: Enhance the biological filtration capacity of your pond by adding beneficial bacteria and enzymes. These microorganisms help break down organic waste, reducing nutrient levels and promoting a healthier pond environment. Choose products specifically formulated for pond use and follow the manufacturer’s instructions for application and dosing.
  • Adequate oxygenation and circulation: Ensure sufficient oxygen levels in the pond by using aeration devices, such as air pumps or waterfalls. Proper circulation prevents stagnant areas that are prone to algae growth and helps distribute oxygen evenly throughout the pond. Consider adding additional aeration devices or adjusting existing ones to improve oxygenation.

Long-term Algae Prevention

Preventing the recurrence of brown algae requires consistent maintenance and vigilance. Consider the following tips for long-term algae prevention:

  • Proper plant selection and maintenance: Choose a variety of aquatic plants that compete with algae for nutrients and shade the pond. Floating plants like water lilies and hyacinths provide shade while absorbing excess nutrients. Submerged plants like anacharis or hornwort help absorb nutrients and oxygenate the water. Regularly remove dead or decaying plant material to prevent nutrient buildup and maintain overall plant health.
  • Avoiding excessive fish feeding and waste accumulation: Feed your koi a balanced diet and only what they can consume in a few minutes. Overfeeding leads to excess nutrients that can fuel algae growth. Monitor your koi’s feeding habits and adjust the amount of food accordingly. Remove any uneaten food using a net or siphon to prevent decomposition and nutrient release into the water. Regularly clean the pond bottom to remove accumulated fish waste.
  • Routine cleaning and debris removal: Regularly clean the pond by removing fallen leaves, twigs, and other debris that can decompose and release nutrients. Skim the pond surface with a net and clean or backwash the filter system as needed. Also, consider using a pond vacuum or siphon to remove sediment and debris settled at the bottom of the pond. Regular maintenance reduces the accumulation of organic matter, which can contribute to nutrient levels and algae growth.
  • Regular water changes: Periodically perform partial water changes to dilute accumulated nutrients and refresh the pond water. Aim for changing about 10-20% of the pond volume every few weeks, depending on the water quality. Use dechlorinated water of similar temperature when performing water changes to minimize stress on your koi.


With proper understanding, prevention measures, removal techniques, and long-term algae prevention strategies, you can effectively get rid of brown algae in your koi pond and maintain a healthy environment for your koi. Regular maintenance, monitoring, and adjustments are essential for sustaining a balanced pond ecosystem. 

Remember to prioritize the well-being of your koi by providing them with a clean and algae-free environment. By following the guidelines in this comprehensive guide, you can ensure a vibrant and beautiful koi pond for years to come.

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