Fleece roller filter: the virtually maintenance-free prefilter

In the best-maintained koi ponds, a new device has taken its place alongside, or instead of, the drum filter: the fleece roll filter, often called a “fleece” filter by koi keepers. Its principle can be summed up in one sentence: water passes through a filter fabric that automatically unrolls as it becomes loaded. No rinse water, no drain connection, remarkable fineness: the fleece filter is the prefilter closest to being “maintenance-free”, provided you accept using a consumable. Here is how it works, what it really costs to use and when it is preferable to a drum filter.

The principle: a fabric that advances all by itself

The filter consists of a housing in which a fleece strip is stretched over a slightly inclined support plate or grid. Water arrives from above, passes through the fabric and exits underneath toward the rest of the system, generally a biological filter. Particles remain on the surface of the fleece and accumulate. Gradually, the water flows more slowly, and its level rises above the fabric. A float or level sensor detects this rise and starts a small motor: a spindle winds up the dirty fleece, pulling an equivalent length of clean fleece from the new roll. The level drops, and the motor stops.

There is therefore no rinse pump, no nozzles, and no dirty water to discharge. All the dirt leaves with the fabric, rolled up and relatively dry. When the new roll is finished, the full core is removed and discarded, and a new roll is installed. Depending on the model, the operation takes a few minutes.

This very simple operation explains the popularity of these filters in installations where connecting to the sewer is difficult: a technical room buried far from the house, a garden without a nearby inspection hatch, or an indoor pond.

A finer actual filtration level than a screen

A drum screen is a fabric with regular mesh: it retains anything larger than its mesh and lets the rest pass through. Fleece, on the other hand, is a tangle of fibers through its thickness. It filters in depth: particles are trapped within the thickness of the fabric, including some smaller than the average space between the fibers. Better still, the layer of dirt deposited on the surface forms a filtering “cake” itself, further refining retention.

The fineness stated by fleece manufacturers is on the order of a few dozen microns, often finer than the standard screens used in drum filters. In practice, the difference is most noticeable with very fine particles that give the water a slight haze. The filter cake is what gives fleece its fineness: fabric that advances too often filters less effectively than fabric that has time to become slightly loaded.

Filter fleece consumption: what makes it vary

This is the question everyone asks, and the honest answer is: it depends enormously on the pond. Two identical installations can consume anywhere from one to five times as much depending on their load. The factors that matter most are gathered here.

As a rough guide, a moderately stocked koi pond uses a few dozen centimeters to a few meters of fleece per day in summer, and almost none in winter. A roll therefore lasts from a few weeks to several months. Record your consumption over a full season: this is the only reliable data for comparison with a drum.

An effective tip: place a coarse prefilter upstream, such as a skimmer basket or screen, to catch leaves and algae before they reach the fleece. The fabric then only has to handle fine particles, which is where it is unbeatable.

How much does a filter fleece cost to use?

The purchase cost is comparable to that of a drum with equivalent capacity, sometimes slightly lower, since there is neither a rinse pump nor a high-pressure spray bar. The real issue is operating cost. It consists of three items:

• Filter fleece rolls: this is the main expense. It varies with consumption and fabric quality. To estimate it, base your calculation on the consumption observed over a season and the price of the roll for your model.
• Electricity: negligible. The motor runs for only a few seconds each time it advances.
• Water: none for the filter itself. This is a real advantage over a drum, which discharges water with every rinse.

On the other hand, the drum uses water and a little electricity for its rinse pump, but no consumables. In a very heavily stocked, generously fed pond, the drum generally becomes more economical in the long term. In a moderately loaded pond, the difference narrows, and the lack of a drain may tip the balance toward fleece. Waste must also be taken into account: used fleece is soiled synthetic textile, which generally goes out with household waste. This is something to consider for those seeking the most resource-efficient installation possible.

Drum or fleece roll: the comparison

Some enthusiasts combine the two: a medium-mesh drum that removes most of the load, followed by a small fleece filter that “polishes” the water. This is a high-end solution, interesting for very large display ponds. To learn more about drum filters, read our article on choosing a drum filter.

Installing a roll filter

Like the drum, the fleece filter is available in gravity-fed and pumped versions. The same basic rules apply: a perfectly adjusted water level in gravity-fed systems, isolation valves on the inlets, pipes with a consistent slope, and electricity protected by a residual-current device and approved by a professional. It has three specific considerations.

1. Space above and around it. You need to be able to open the lid, remove the full core and slide in a new roll without contortions. In a cramped technical room, this is often what is missing.
2. The level of the tray. A housing that tilts causes water to flow over one side of the fabric and leads to lateral overflow, with unfiltered water bypassing the fleece.
3. An overflow or safety feature. If the roll is used up or the motor jams, the water level rises. Depending on the model, an alarm provides a warning, a bypass lets unfiltered water through, or the pump is switched off. Check which of these devices your filter has and test it.

Like any pre-filter, it must be followed by a biological stage. The fleece traps particles; it does not convert ammonia or nitrites. And because dirt leaves the circuit very quickly, the biological stage placed afterward stays clean and operates under ideal conditions. Our article on Japanese filter mats for filtration presents a common biological medium used in these stages.

Settings and maintenance

The main adjustment is the trigger height. A float set too low makes the fabric advance at the slightest trace of deposit: consumption soars and filtration becomes less fine, since the cake never has time to form. Set too high, it lets the water rise to the bypass. Adjust it by observing operation for a few days, then make gradual changes.

Maintenance is limited to changing the roll, occasionally cleaning the tray and sensor (a biofilm layer can distort detection), and checking the motor and seals once a year. In winter, when feeding stops, consumption drops sharply; some owners take advantage of this to make a partially used roll last until spring.

Common mistakes

• Setting the sensor too low: consumption increases severalfold and filtration fineness deteriorates.
• Sending string algae directly onto the fleece: it covers the fleece within a few hours.
• Choosing a model with barely sufficient flow capacity: the fabric advances continuously during feeding times.
• Forgetting to stock spare rolls: a roll running out on a Friday evening in June means a weekend without prefiltration.
• Removing the biological stage: water quality is not just about clarity.
• Using a generic fleece not designed for the system: its width, thickness, and mechanical strength must match the model; otherwise, the fabric may tear or drift.

Generic practical cases

Common case: a 15 m³ pond in a garden without a drain nearby. Around ten koi, with moderate feeding. A pumped fleece filter, followed by a chambered biological filter, avoids having to dig a drain and produces very clear water. Consumption remains reasonable.

Common case: a large, heavily stocked pond already equipped with a drum filter. The owner wants even greater clarity. Rather than replacing the drum, a small fleece filter is added downstream to handle part of the flow. The water becomes clearer without fabric consumption skyrocketing, since the drum already retains most of the debris.

Common case: a fleece filter that “eats” a roll in ten days. On inspection, we discover overfeeding and string algae being sucked in by the skimmer. A coarse prefilter upstream, more digestible food, and raising the sensor by a few millimeters cut consumption in half.

To understand why fine prefiltration improves the rest of the system, also read why you should filter your pond water.

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Frequently asked questions

— Can used fleece be washed and reused?

It is not designed for that. The fabric deforms, loses its structure, and filters much less effectively after washing. It is designed as a consumable.

— Does a fleece filter work in winter?

Yes, in a frost-free room. As the load drops significantly when feeding stops, the fabric advances very little and consumption becomes negligible.

— Fleece filter or drum for a 30 m³ pond?

If the pond is heavily stocked and a sewer is accessible, the drum is generally more economical. Without a possible discharge, or for a moderate load, fleece is an excellent option.

— Is a biological filter still needed after a roll filter?

Yes. The fleece removes particles, but the bacteria in the biological stage convert ammonia and nitrites.

In summary

• The roll filter automatically advances clean fleece when the fabric becomes clogged.
• It filters very finely, without rinse water or discharge to the sewer.
• Its operating cost mainly depends on fleece consumption, and therefore on the pond's load.
• A coarse prefilter upstream and a well-adjusted sensor significantly reduce consumption.
• Like any prefilter, it must be followed by a biological stage.

Last updated: October 3, 2026.