Pond filter brushes: how many to use and how to install them

In a multichamber gravity-fed filter, the first chamber is often filled with brushes. They may seem simple, but their number and arrangement make a real difference to water clarity and how often the rest of the filter needs cleaning. Here's how to calculate the number and install them correctly.

What are brushes used for in a filter?

The brushes provide the mechanical filtration stage. Positioned upstream of the biological section, sometimes after a screen filter, they retain waste, plant debris and algae clumps removed by the UV lamp. Without this stage, foams and biological media quickly become clogged and the filter loses its effectiveness.

Quality matters: a stainless steel central wire and rigid polypropylene fibres retain their shape for years. The denser and firmer the fibres, the better the pre-filtration.

Vertical or horizontal installation?

— Vertical installation (recommended)

Water enters through the bottom of the tank and exits on the opposite side, at the top. As it rises, it passes through the entire mass of fibres. Particles are retained and the heaviest settle at the bottom. This is the most effective configuration.

— Horizontal installation

The inlet and outlet are at the same level. Water passes through the brushes without moving upward. Mechanical filtration is slightly less effective, but this arrangement remains useful when the tank height is limited.

How many brushes should you use?

The calculation is based on the tank's internal dimensions.

1. Height: leave 10 to 20 cm at the bottom for settling. If your brushes have a mounting ring, allow for around 7 cm that cannot be used.
2. Length and width: divide each dimension by the brush diameter, allowing for an overlap of at least 2 cm.
3. Multiply the number of rows by the number of brushes per row.

Example: a 75 cm long tank with brushes 20 cm in diameter. You need 4 brushes per row, interlocking with one another. With 3 rows across the width, that makes 12 brushes.

Maintaining the brushes

• Open the bottom drain valve to remove sludge regularly.
• Take out and rinse the brushes in a bucket of pond water, a few at a time.
• Never use tap water or a pressure washer: you would destroy the beneficial bacteria.
• In spring, carry out a more thorough cleaning before the season resumes.

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To preserve your filter's biological balance, read Why and how to add bacteria to a pond?.

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Frequently asked questions

— Can brushes be replaced with foam?

In the first chamber, brushes are preferable: they clog much less quickly and are easier to clean than fine foam.

— Which diameter should you choose?

Diameters from 10 to 20 cm are the most common. The smaller the diameter, the more brushes are needed, but the more uniform the mass of fibers.

— Do the brushes need to be replaced?

Quality brushes last for many years. Replace them when the fibers are crushed or deformed.

In summary

• The brushes provide the filter's mechanical prefiltration.
• Install vertically, with a minimum 2 cm overlap and a clear bottom for settling.
• Clean with pond water, in small sections.

Last updated: September 27, 2026.