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Biological filter media

By the koi.wiki editorial team · Updated

Key facts: Biological filter media is the non-corroding substrate—such as plastic media, matting, sand or beads—inside a biofilter on which nitrifying bacteria colonize to convert fish waste into less toxic compounds.[1][2]
Biological filter media provides surfaces on which nitrifying bacteria establish a biofilm, performing the nitrification step of the nitrogen cycle in a koi pond. According to the Southern Regional Aquaculture Center, a biofilter consists of non-corroding material such as plastic, fiberglass, ceramic or rock with large amounts of surface area that nitrifying bacteria cells can colonize.[1] Bacteria convert ammonia excreted by fish into nitrite, and then into the less toxic nitrate, with the whole process requiring oxygen.[1][3]In koi ponds, biological filters are often combined with mechanical filtration, which removes solids before they can clog the biological media.[2] Media types used in pond and aquaculture settings range from simple foam and plastic media in submersible in-pond filters to specialized moving bed media, fluidized sand beds and floating bead filters.[4][5]

Surface area and bacterial colonization

The amount of bacteria a biofilter can support depends largely on its specific surface area (SSA), the surface area available per unit volume of media. The Southern Regional Aquaculture Center notes that the more surface area available, the more bacteria cells can be grown and the greater the nitrification capacity, allowing higher feed rates to be supported; a biofilter with higher SSA will be more compact than one with lower SSA.[1] However, media with very high SSA can become clogged with bacteria, so a balance must be struck between high surface area and an operationally reliable design.[1]Nitrifying bacteria grow on all wet surfaces of a system, not just the dedicated biofilter media, including the insides of pipes and tank walls, and they follow a continual cycle of growing, maturing, dying and sloughing off, being replaced by new cells.[1] In aquariums, the largest bacterial populations typically inhabit the filter, and the population of nitrifying bacteria is limited by the available physical space, including all submerged surfaces.[8]

Types of biological media

Common pond media listed by the Southern Regional Aquaculture Center for recirculating aquaculture systems include gravel, sand, plastic beads, plastic rings, plastic tubes and plastic plates.[7] A simple in-pond filter design used in water gardens uses plastic media and foam surrounding or connected to a submersible pump, acting as both mechanical and biological filtration.[4][6]Moving bed reactors use larger plastic media, about 6 to 13 millimeters (roughly 1/4 to 1/2 inch), specifically manufactured to provide protected surfaces for fixed-film bacteria; the media sits in a constantly aerated tank.[5] A fluidized bed suspends a bed of sand in an upflowing current of water, providing very high surface area, with each sand grain gently bumping adjacent grains to abrade excess biofloc; a well-designed fluidized bed can achieve the highest volumetric conversion rates among biofilter types.[5] Floating bead filters use polyethylene beads of about 3 millimeters (1/8 inch), which can function as combined bioclarifiers (solids capture plus biofiltration) or, in dynamic form, as continually moved biofilters using shaped polyethylene or small styrene beads.[5]More general aquarium filtration sources note that materials with greater surface area provide both mechanical and biological filtration, including synthetic sponges, ceramic and sintered glass products, igneous gravels and plastic "bioballs," which are best used for biological filtration.[9] A koi pond-focused source lists moving bed filters, bakki showers/trickle filters, sand filters, cross-flow filters and bead filters among biofilter types, distinguishing between aerated and non-aerated designs.[2]

Matting and simple pond media

Matting is referenced among mechanical filter types for koi ponds, alongside vortex units, brushes, sand and gravel, sieve screens and settlement chambers, with the role of trapping solids before they reach the biological filter.[2] Texas A&M AgriLife Extension describes the under-gravel filter as using gravel as a mechanical filter that becomes colonized by bacteria, though these are prone to clogging and require laborious cleaning; an up-flow bio-filter instead uses a coarse media colonized by bacteria and contains a separate sediment basin to collect debris.[6]Water-garden guidance also notes that some bead-type biofilter materials, such as polystyrene beads, have shown initial resistance to bacterial colonization, possibly due to surface additives such as flame retardants; this resistance is usually short-lived and most operators work through the problem.[1] New media in general can be harder to colonize initially because of a slick, shiny surface, with subsequent cycles usually starting more quickly once the first bacterial growth is established.[1]

Conditions that support nitrification

Nitrification is an aerobic process that is absolutely dependent on the availability of oxygen and is also sensitive to alkalinity and pH.[5][7] As ammonia is converted to nitrate, hydrogen ions are produced, consuming alkalinity and lowering pH; a pH below 7.0 will reduce the activity of nitrifying bacteria, and levels below 4.5 are dangerous to fish.[7] Texas A&M AgriLife Extension states that bacterial biofiltration systems operate best at a pH of 7 to 8 and an alkalinity greater than or equal to 50 ppm, though alkalinity is consumed over time and must be replenished.[6]The Southern Regional Aquaculture Center recommends raising alkalinity to about 150 mg/L initially to support Nitrosomonas growth, noting that establishing Nitrobacter has had better success at a higher alkalinity of about 200 to 250 mg/L, after which alkalinity can be allowed to decline to operational levels.[1] A new biofilter generally requires six to eight weeks to build up sufficient bacteria to effectively reduce ammonia and nitrite levels, according to University of Florida IFAS Extension.[3]

Maintenance, cleaning and disruption of media

Because filtration and biomass growth cause matter to accumulate in filter media, biological filters are subject to bioclogging and flow channeling, which can be controlled with backwash steps using air and/or water.[10] In aquarium practice, best practice is to flush mechanical filters using compatible water to dislodge organic material while preserving bacteria populations, and a recommended safe practice is to clean only half of the filter media at a time so the remaining bacteria can repopulate the cleaned half.[8]With moving bed or mixed bed biofilters, bacteria may be slow to colonize because of excessive agitation or aeration, which mechanically abrades cells trying to adhere; reducing aeration and water flow through the bed can help, with aeration gradually increased as more bacteria become established.[1] Bath antibiotic treatments are not recommended in systems where treated water will contact the biological filter, since antibiotics may kill or inhibit nitrifying bacteria.[11] Similarly, ozone is non-selective and can react with beneficial microorganisms, though a properly designed and contained system does not necessarily eliminate beneficial bacteria throughout the pond or biological filter, according to the US dealer Kodama Koi Farm.[12]

Starting and supplementing bacterial colonies

Starting a biofilter means managing and controlling the seeding of nitrifying bacteria cells onto the media.[1] Bacteria can be introduced with water or bits of biofilter media from an already operating system, with pond sediment or barnyard soil, or with small numbers of hardy "starter" animals; whichever method is used, there is a risk of introducing pathogens, so the method should be evaluated as part of overall biosecurity planning.[1] A "cold start" method stocks fish without an activated biofilter, requiring operators to manage resulting ammonia and nitrite spikes through water exchanges and reduced feeding.[1]Commercial preparations of cultured nitrifying bacteria are also available as dry powders or liquid suspensions and, while not essential, can shorten the time needed to establish a biofilter; the US dealer Kodama Koi Farm markets nitrifying bacteria products intended to strengthen biological filtration, particularly when starting a new pond or filter, opening ponds in spring, after medication treatments, or following major water changes and filter cleaning.[1][13]

Common questions

What makes good biological filter media?
According to the Southern Regional Aquaculture Center, good media has a large specific surface area (surface area per unit volume) to support bacterial colonization while remaining operationally reliable and resistant to clogging.[1]
How long does it take for biological media to become effective?
A new biofilter generally requires six to eight weeks to build up sufficient nitrifying bacteria to effectively reduce ammonia and nitrite levels.[3]
Can cleaning filter media harm the bacteria colony?
Yes; best practice in aquariums is to flush mechanical filters with compatible water rather than tap water, and to clean only part of the media at a time so remaining bacteria can repopulate the cleaned portion.[8]
Do antibiotics or ozone damage biological media?
Bath antibiotic treatments can kill or inhibit nitrifying bacteria if the treated water contacts the biofilter.[11] Ozone is non-selective and can react with beneficial bacteria, though a properly designed, contained system does not necessarily eliminate them throughout the pond or filter, according to the dealer Kodama Koi Farm.[12]
Why do some bead media resist bacterial colonization at first?
Some polystyrene bead material has shown initial resistance to colonization, possibly due to surface additives such as flame retardants, though this resistance is usually short-lived.[1]

References

  1. ^ SRAC Publication No. 4502: How to Start a Biofilter. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4502%20How%20to%20Start%20a%20B…. Retrieved November 2026.
  2. ^ Koi pond (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Koi_pond. Retrieved November 2026.
  3. ^ UF/IFAS EDIS FA031: Ammonia in Aquatic Systems. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA031. Retrieved November 2026.
  4. ^ SRAC Publication No. 435: Water Gardens. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/435%20Water%20Gardens.pdf. Retrieved November 2026.
  5. ^ SRAC Publication No. 453: Recirculating Aquaculture Tank Production Systems: A Review of Current Design Practice. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/453%20Recirculating%20Aquacultu…. Retrieved November 2026.
  6. ^ Ornamental Fish Ponds and Water Gardens in Texas (Texas A&M AgriLife Extension). Texas A&M AgriLife Extension. extension.rwfm.tamu.edu/wp-content/uploads/sites/8/2013/09/Ornament…. Retrieved November 2026.
  7. ^ SRAC Publication No. 451: Recirculating Aquaculture Tank Production Systems: An Overview. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/451%20Recirculating%20Aquacultu…. Retrieved November 2026.
  8. ^ Fishkeeping (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Fishkeeping. Retrieved November 2026.
  9. ^ Aquarium filter (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Aquarium_filter. Retrieved November 2026.
  10. ^ Biofilter (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Biofilter. Retrieved November 2026.
  11. ^ UF/IFAS EDIS FA084: Use of Antibiotics in Ornamental Fish Aquaculture. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA084. Retrieved November 2026.
  12. ^ Ozone vs. UV for Koi Ponds: Which Is Better?. Kodama Koi Farm (koi dealer). kodamakoifarm.com/ozone-vs-uv-for-koi-ponds/. Retrieved November 2026.
  13. ^ How to Pick the Right Pond Water Conditioner. Kodama Koi Farm (koi dealer). kodamakoifarm.com/how-to-pick-pond-water-conditioner/. Retrieved November 2026.