Types of Bio Filters in Fish Pond

Types of Bio Filters in Fish Pond

A thriving fish pond is a beautiful addition to any garden. But maintaining a healthy ecosystem for your finned friends requires more than tossing some fish food in occasionally.  A well-functioning filtration system is crucial to a pond’s health, and a biofilter sits at the heart of this system.

Biofilters are the biological workhorses of pond filtration. They don’t physically trap debris like mechanical filters, but instead, house colonies of beneficial bacteria that break down harmful waste products produced by fish and decaying organic matter. Understanding the different types of biofilters available will help you choose the right one for your pond and keep your fish happy and healthy.

The Nitrogen Cycle: The Foundation of Biofiltration

Before diving into biofilter types, let’s take a quick detour to understand the nitrogen cycle. Fish excrete ammonia, a toxic substance, as waste. Fortunately, beneficial bacteria in your biofilter come to the rescue! These bacteria come in two main types:

  • Nitrosomonas bacteria: These guys convert ammonia, which is highly toxic to fish, into nitrite, another harmful compound.
  • Nitrospira bacteria: Taking over from the Nitrosomonas crew, Nitrospira bacteria convert the nitrite into nitrate, a much less toxic form of nitrogen that plants can readily absorb as a nutrient.

This process, known as nitrification, is essential for maintaining healthy water quality in your pond. Biofilters provide the ideal environment for these beneficial bacteria to thrive, ensuring a constant breakdown of harmful ammonia and nitrite.

Biofilter types : Choosing the Right One for Your Pond

Now that we understand the importance of the nitrogen cycle and the role biofilters play, let’s explore the different types of biofilters available:

Submerged Media Filters:

These are the most common types of biofilter and come in various shapes and sizes. Submerged media filters consist of a chamber filled with media that provides a large surface area for beneficial bacteria to colonize.  Common media options include:

Bio balls: These plastic balls have a large surface area that allows for ample bacterial growth. They are relatively inexpensive and easy to clean.

Bio mats/rings: These interlocking mats or rings offer a complex structure with a high surface area, perfect for bacterial colonization. They are also easy to clean and maintain.

Pot scrubbers: These cylindrical scrubbers made of recycled plastic have a rough surface that provides excellent surface area for bacteria. They are durable and relatively inexpensive.

Submerged media filters are typically placed in a canister or box outside the pond and connected to a pump that circulates pond water through the media.

Pros: Simple to set up and maintain, affordable, good for various pond sizes.

Cons: Can clog with debris if not paired with a pre-filter, and may require more frequent cleaning for heavily stocked ponds.

Fluidized Bed Filters:

These high-tech biofilters offer superior biological filtration capabilities. They utilize a chamber filled with lightweight media, typically small plastic beads. A powerful pump forces water upwards through the chamber, creating a fluidized bed where the media particles move freely. This constant movement provides excellent oxygenation and prevents the media from clogging, maximizing the surface area for beneficial bacteria.

Pros: Highly efficient biological filtration, requires less cleaning than other types, suitable for heavily stocked ponds.

Cons: More expensive than submerged media filters, requires a powerful pump and can be complex to set up.

Bead Filters:

These biofilters offer a good balance between efficiency and affordability. They use a chamber filled with small plastic beads similar to fluidized bed filters, but the water flow is not strong enough to fully fluidize the media. Bead filters provide a good surface area for bacterial growth and are relatively easy to maintain.

Pros: Efficient biological filtration,  easier to set up and maintain than fluidized bed filters, suitable for various pond sizes.

Cons: Not as efficient as fluidized bed filters, may require more cleaning than fluidized bed filters for heavily stocked ponds.

Moving Bed Bioreactors (MBBR):

Similar to bead filters, MBBRs utilize a chamber filled with media for bacterial growth. However, the press in MBBRs is specifically designed to have a large surface area and slow settling rate. This allows the media to move freely in the water column as it is circulated by a pump, maximizing contact between the bacteria and the water.

Pros: Highly efficient biological filtration, requires minimal cleaning,

Types of Bio Filters in Fish Pond

Choosing the Right Biofilter Types for Your Pond

With so many biofilter options, selecting the right one for your pond can feel overwhelming. Here are some key factors to consider:

  • Pond size and fish population: Larger ponds and those with a higher fish population will require a more efficient biofilter.  Fluidized bed filters and MBBRs are good choices for heavily stocked ponds, while submerged media filters or bead filters can handle smaller or lightly stocked ponds.
  • Maintenance: Consider how much time and effort you will dedicate to cleaning your biofilter. Submerged media filters and bead filters are generally easier to clean than fluidized bed filters or MBBRs.
  • Budget: Biofilters range in price depending on their type and complexity. Submerged media filters are typically the most affordable option, while fluidized bed filters and MBBRs are more expensive.
  • Aesthetics: Undergravel and pond bog filters offer a more natural look as they are hidden from view. However, submerged media filters and canister filters can be quite visible.

Additional Considerations for Biofilter Success

Once you’ve chosen your biofilter, here are some additional tips to ensure it functions optimally:

  • Proper sizing: A correctly sized biofilter will have enough surface area for the beneficial bacteria to thrive and handle the waste load from your fish.
  • Regular cleaning: Biofilter media can clog with debris over time, reducing its effectiveness. Clean your biofilter media regularly according to the manufacturer’s instructions. Avoid cleaning everything at once, as this can disrupt the delicate balance of bacterial colonies.
  • Water flow: Ensure adequate flow through your biofilter to provide oxygen to the beneficial bacteria and prevent clogging.
  • Maintain good water quality: Regular water changes and testing are still essential, even with a biofilter. A biofilter works best when ammonia and nitrite levels are low.


Biofilters are the unsung heroes of healthy fish ponds. By understanding the different types of biofilters available and choosing the right one for your pond, you can create a thriving ecosystem for your fish and enjoy the beauty of your crystal-clear pond for years.

Tony Augustine

About the author

In 2012, I helped establish KJA & Sons and have been deeply engaged in many swimming pool initiatives ever since. My expertise lies in designing filters for chlorine-free swimming pools, as well as living water gardens, koi fish ponds etc.