Filter media for ponds: foam, ceramic, K1, Japanese mats - order and quantities

Two identical filters can produce very different water depending on what is put inside them and in what order. Good filtration uses mechanical media arranged from coarsest to finest, followed by biological media sized according to the amount of food distributed, rather than solely according to the pond volume. Here's how to choose between foams, ceramics, K1 and Japanese mats, how much to use and how to maintain them without losing the bacteria.

Mechanical vs biological role

Not all filter media do the same job. Some trap particles: this is mechanical filtration. Others provide a surface to which nitrifying bacteria attach, transforming the toxic ammonia released by fish into nitrites, then into much less toxic nitrates: this is biological filtration. Most media do a little of both, but each has a primary purpose.

The basic principle can be summed up in one sentence: first remove the particles, from coarsest to finest, then provide the bacteria with the cleanest water possible and a large, well-oxygenated surface. Biological media that receives sludge becomes clogged, lacks oxygen and loses its effectiveness. Mechanical media that is too fine and placed first becomes blocked within a few days.

A word about the concept of surface area. Manufacturers often advertise a "specific surface area", in square metres per cubic metre of media. This figure only makes sense if that surface remains accessible to water and oxygen. Ceramic media with microscopic pores clogged by silt no longer provides the advertised surface area. It is better to use media with a more modest surface area that remains well irrigated than theoretically more efficient media that becomes suffocated.

Foams: grain sizes and uses

Filter foams are blocks or sheets of open-cell polyurethane foam. Their fineness is expressed in ppi, meaning “pores per inch,” the number of pores per inch. The higher the number, the finer the foam.

Foams have three advantages: they are lightweight, inexpensive, and easy to cut to size. Their drawback is their appetite for sludge. In a filter operated without a prefilter, they require frequent rinsing during the season. Good-quality foam retains its elasticity for several seasons; when it becomes brittle or starts to crumble, it is time to replace it.

Multichamber filters often use a set of foams in different colors, with each color corresponding to a grain size. Follow the order specified by the manufacturer: reversing a fine foam and a coarse foam is enough to clog the filter within a few days.

Ceramics, balls, and lava rocks

These static, heavy media remain in place in the chamber. They are mainly biological.

• Porous ceramic rings or balls offer a structure rich in pores of varying sizes. They hold up well over time and do not deform. Their weakness: the finest pores become clogged if the water arrives loaded with particles.
• Lava rocks are inexpensive and very irregular, giving them a good surface area. They are heavy, and their edges can damage a liner or a plastic filter wall if they are poured in carelessly. Rinse them before use to remove dust.
• Static plastic media in the form of rings or perforated balls are lightweight, easy to clean, and not very prone to clogging, but their surface area is smaller than that of ceramic media.

These media are placed in chambers where the water has already been cleared of most of its particles. They are generally placed in mesh bags so they can be removed easily during maintenance.

K1 and moving media

K1 and its equivalents, such as propeller-shaped media, are small polyethylene cylinders whose inner surface, protected from friction, is colonized by bacteria. Their density is close to that of water. They can therefore be used in two ways:

• In a moving bed, continuously agitated by air injected from the bottom. Friction between the pieces removes excess biofilm: the media self-cleans and remains highly effective.
• In a static bed, without agitation. K1 then also retains fine particles and plays a mechanical role, at the cost of occasional cleaning by agitation.

The moving bed is now one of the most effective biological solutions for a koi pond, provided it is supplied with enough air. The subject deserves a separate article devoted to sizing and aeration of moving-bed filters.

Japanese mat

The Japanese mat is a matrix of rigid plastic filaments intertwined into a highly aerated structure. Water passes through it without significant resistance, particles adhere to it, and bacteria colonize it. It therefore forms the link between mechanical and biological filtration.

Its main advantage is that it hardly clogs and can be rinsed in a few seconds with a jet of pond water without losing its shape. It is often placed after brushes or a prefilter, in tightly packed vertical sheets or cassettes. For choosing the thicknesses and assembling it, see our guide to Japanese mats.

Calculate the quantity based on the fish load

The amount of biological media depends less on the volume of the pond than on what is added each day: food. It is the proteins in the food that, once digested, are converted into ammonia excreted through the gills. In aquaculture, ammonia nitrogen production is conventionally estimated from the feed ration and its protein content.

Here is a simplified three-step method, with an example. The values are orders of magnitude intended to establish an initial sizing, not standards.

1. Estimate the maximum daily ration. In summer, koi are often fed approximately 1 to 2% of their body weight per day. Example: ten koi of around fifty centimeters in length weigh approximately 15 to 20 kg in total. At 1.5%, the ration is around 250 to 300 g per day.
2. Deduce the ammonia produced. A commonly used aquaculture rule estimates ammonia nitrogen production at approximately 9% of the mass of protein distributed. With 280 g of feed containing 40% protein, this gives approximately 112 g of protein, or around 10 g of ammonia nitrogen per day.
3. Convert to surface area, then to media volume. Well-oxygenated biological media at around twenty degrees treats on the order of a few tenths of a gram of ammonia nitrogen per square meter of active surface area per day. Using a cautious value, about thirty square meters of genuinely active surface area are needed, doubled to maintain a margin in spring and autumn, when the bacteria work more slowly.

This calculation shows two things. First, food is the real driver of filter load: doubling the rations means doubling the work required of the bacteria. Second, biological capacity is rarely oversized in entry-level filters sold for a given pond volume. These stated volumes often assume a lightly stocked pond. For a koi pond, planning generously is the rule.

Filling order for a multi-chamber filter

In a multi-chamber filter, the water must pass through the media in a logical order: from coarsest to finest for the mechanical section, then the biological section, and finally, if needed, a polishing stage. Here is a typical example for a four-chamber pumped filter:

In a gravity-fed system with a drum filter, the principle is the same, simplified: the drum handles the mechanical filtration, and the following chambers hold almost exclusively biological media, often K1 in a moving bed, sometimes supplemented by a chamber of static K1 or mats.

Two mistakes are common: placing ceramic media in the first chamber, where it becomes a sludge trap, and leaving preferential paths, such as media bags that do not fill the entire section. The water then flows around them instead of passing through.

Maintenance and replacement without losing the bacteria

Nitrifying bacteria live attached to the media, in the biofilm. Overly vigorous cleaning removes some of them. A few simple rules limit the losses:

• Rinse in pond water, never in chlorinated tap water or with a household cleaning product.
• Never clean everything on the same day. Alternate: one chamber or half the foams, then the rest one to two weeks later.
• Aim for “clean,” not “new.” Rinsed foam retains a brownish color: this is normal; it is the biofilm.
• Replace gradually. Replace worn foam by half, never the entire set at once. Ceramic media can last for many years; rinse it only if it is really clogged.
• Restore the bacterial flora if necessary. After a major replacement or treatment, adding starter bacteria helps rebuild the colony.

The role of bacteria is detailed in our complete guide to pond bacteria. To choose the filter that will hold these media, also see how to choose the right filter for a garden pond.

Read also

To learn more about Japanese filter mats: Choosing and using Japanese filter mats for pond filtration.

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Frequently asked questions

— In what order should the filter media be placed?

From coarsest to finest for the mechanical section (brushes, coarse-grain foams, medium foams), followed by biological media (K1, ceramics), and optionally a fine polishing foam at the outlet.

— How much K1 is needed for a koi pond?

This depends mainly on the amount of food. For around ten adult koi, approximately 100 to 150 liters of K1 in a well-aerated moving bed is a common starting point, to be adjusted according to the load.

— Should ceramic media be replaced?

Rarely. Quality ceramic media lasts for many years; rinse it with pond water when it is loaded with sludge.

— Can foams be cleaned with tap water?

It is best avoided: chlorine in tap water can damage the bacteria. Rinse them in a bucket of pond water, half at a time.

In summary

• Mechanical filtration first, from coarse to fine; biological filtration afterward, with the cleanest water possible.
• Foams: approximately 10 to 45 ppi, to be placed in the specified order.
• Ceramics and lava rock: biological media that should be protected from clogging.
• K1: highly effective in an aerated moving bed, also useful in a static bed.
• The amount of media is calculated based on the food distributed, with a margin.
• Change the pond water by half to preserve the bacteria.

Last updated: October 3, 2026.