Pond pre-filter: vortex, curved screen, brushes, or drum - which one should you choose?

Biological filtration is only as good as the stage before it. If leaves, waste, and sludge reach the biological media, they become clogged, oxygen levels collapse, and the water becomes loaded with pollutants. The prefilter removes these particles before they decompose: the choice between a vortex, curved screen, brushes, drum, or roll filter depends on the desired filtration fineness, the maintenance time you are willing to accept, and your budget. Here is how to compare these solutions.

Why prefilter before biological filtration

A pond filter performs two very different functions. The first is mechanical: retaining particles, from leaves to waste. The second is biological: providing a surface for bacteria that transform ammonia into nitrites and then nitrates. The problem is that these two functions interfere with each other when they are entrusted to the same medium.

Organic waste trapped in the biological media continues to decompose. This decomposition consumes oxygen, releases ammonia and phosphates, and feeds heterotrophic bacteria that take the place of nitrifying bacteria. The filter becomes clogged, water flows through preferential paths, and efficiency drops. A good prefilter removes particles while they are still intact, before they dissolve into the water and overload the biological filtration.

The prefilter is therefore the first chamber or device that the water encounters. It is not intended to “make the water clear” on its own, but to protect everything downstream. The more finely it filters, the more often it must be emptied, and the less pressure the biological filtration has to work under.

There are five main types of prefilters. They differ in the fineness of what they retain, the maintenance they require, and their price.

The vortex: settling by rotation

The vortex is a cylindrical tank with a conical bottom. Water enters tangentially, creating a slow swirling motion. The heaviest particles lose their speed, slide along the walls and gather at the center of the cone, from where a drain valve removes them. The water, with part of its load removed, exits at the top toward the next stage.

The vortex has no moving parts and consumes no energy. For years, it was a basic component of filtration systems for koi ponds. Its limitations are well known:

• It mainly retains heavy, settleable particles, and far fewer fine suspended particles;
• It requires a large volume and works mainly in gravity-fed systems, with a low water velocity;
• It is useless if it is not purged: the bottom valve must be opened regularly, often every week during the season.

A vortex can be greatly improved by adding brushes or a brush ring, which catch some of the particles that do not settle. This is a robust solution for a budget-conscious gravity-fed system.

The curved screen (sieve): principle and maintenance

The curved screen is a stainless-steel surface made of very closely spaced profiled wires, mounted in an arc. Water enters at the top of the screen, flows over it and passes through the slots. Particles larger than the spacing between the wires remain on the surface and move down toward a collection trough. Common pond screens have slots of around 200 to 300 microns.

Its advantages are real: no electricity consumption, much finer filtration than a vortex, a compact footprint, and versions suitable for both pumped and gravity-fed systems. Its weak point is maintenance.

• The screen gradually becomes covered with a greasy film and biofilm, which clog the slots. Water no longer passes through; it runs down the surface and goes through the overflow.
• It needs to be brushed regularly, sometimes every day in peak season when the pond is heavily loaded. A soft brush is sufficient; brush in the direction of the wires.
• The collection tray is emptied through a valve at regular intervals.
• In hard-water regions, limescale deposits can occasionally be treated with a suitable product, with the screen removed and rinsed before being put back in place.

Some screens are mounted on an axle and can be flipped over, reversing the flow direction and dislodging some of the deposits. Over time, the curved screen offers excellent value for money, provided you are willing to carry out this daily or almost daily task.

Brushes: simple and economical

A brush chamber is the simplest form of prefiltration. Cylindrical brushes with plastic bristles, suspended vertically side by side, form a forest that water must pass through. Coarse debris catches on them, then some of it falls to the bottom of the chamber, where it is removed.

Brushes retain only relatively large particles. On the other hand, they almost never become clogged to the point of blocking water flow, making them reliable protection even if they are neglected for a few weeks. They are also colonized by bacteria and play a secondary biological role.

To use them properly:

• Pack them tightly. If you can see water passing between the brushes in a chamber, they filter almost nothing. They must touch one another.
• Choose a length suited to the height of the chamber, so that water does not pass underneath the brushes.
• Rinse them in pond water, a few at a time, shaking them in a bucket. Do not use a pressure washer or chlorinated tap water on all the brushes at once.
• Empty the bottom of the chamber regularly: whatever falls from the brushes remains organic waste.

Brushes also have their place in a pumped multichamber filter, in the first chamber, in front of foams. They retain the largest debris and extend the intervals between foam rinses.

The drum filter: automation

The drum filter is the most advanced prefilter for a koi pond. Water enters a horizontal cylinder covered with a fine screen, usually made of polyester or stainless steel, and passes through this screen from the inside out. The particles remain inside the drum.

When the screen becomes clogged, water passes through it less easily and the level drops in the outlet chamber. A float or sensor detects this drop and triggers the cleaning cycle: the drum rotates, a spray bar projects pressurized water through the screen from the outside, and the dislodged waste falls into a gutter connected to the sewer.

Its strengths:

• high fineness, commonly around 40 to 100 microns depending on the selected mesh;
• waste discharged from the circuit within a few minutes of arriving, before it dissolves;
• reduced maintenance limited to checks: nozzle cleanliness, mesh condition, and rinse pump operation.

Its limitations:

• a high purchase price;
• a power supply and a nearby sewer outlet;
• rinse water consumption, which increases if the mesh is very fine or if the pond is heavily loaded;
• optimal operation in gravity-fed mode, although pumped models are available.

Sizing is based on the maximum flow rate accepted by the drum, for the selected fineness. A finer mesh retains more but reduces this flow rate and increases the number of rinses. Our guide to choosing a drum filter details these criteria.

The roll or paper filter

Less widespread, the roll filter uses a non-woven fabric belt, comparable to thick filter paper, stretched between two rolls. Water passes through the belt, which retains very fine particles. When it becomes clogged, a level sensor advances the roll: a new section is presented and the dirty section winds onto a take-up roll, along with the waste.

The filtration obtained is very fine, often finer than that of a drum filter, and the unit does not consume rinse water. In return, the rolls are consumables that must be replaced, representing a regular cost, and the waste is disposed of with the used roll. This type of filter is suitable for owners seeking maximum water clarity and willing to accept this operating cost.

Fineness/maintenance/cost comparison table

The indicated fineness levels are approximate and intended to guide the choice. They depend on the model, the selected mesh or screen, and the actual flow rate.

Another type of media, often used as a complement, is Japanese matting. It is generally placed after the prefilter, as it combines mechanical filtration with a biological role. We have dedicated an article to it: choosing and using Japanese matting.

Which combination for which pond

There is no universal prefilter. The right choice depends on the fish load, the design (pumped or gravity-fed), the budget and the time available. Here are the combinations we recommend most often.

Common case: a 12 m³ pumped pond with around ten koi, equipped with a foam filter. The foams need rinsing every week from June onward, and the water remains slightly cloudy. Adding a curved screen before the filter changes the routine: the foams only clog once a month, and the better-oxygenated biological media catch up. The daily task simply shifts to the screen, which is quicker to brush than removing a set of foams.

Another common case: a gravity-fed system with a vortex and brushes, on a 25 m³ pond. The owner flushes it irregularly, sludge rises and the water becomes loaded with phosphates. Before investing in a drum filter, the first improvement is often to schedule flushing, or even install an easy-to-operate valve, then supplement it with Japanese matting. The drum can come later if time is limited.

Read also

If the drum appeals to you: How to choose a drum filter for a pond?.

READ THE ARTICLE

To put the prefilter in the context of the overall installation, also refer to our guide to choosing a pond filter.

Frequently asked questions

— Is a prefilter really necessary?

As soon as the pond houses a significant number of fish, yes. Without a prefilter, waste decomposes in the biological media, clogs them and consumes the oxygen the bacteria need.

— What mesh size should you choose for a drum filter?

A mesh of around 60 to 100 microns is suitable for most ponds. Finer mesh captures more particles but reduces the permissible flow rate and increases rinsing frequency.

— How often should a curved screen be brushed?

During peak season, on a heavily stocked pond, often every day. In spring and autumn, two to three times a week is generally sufficient.

— Vortex or curved screen, which should you choose?

The curved screen captures finer particles and takes up less space. The vortex is more tolerant of missed maintenance and does not clog, but it lets fine particles pass through.

In summary

• The prefilter protects the biological filter by removing waste before it decomposes.
• Vortex and brushes: economical, robust, but limited to coarse particles.
• Curved screen: good fineness without electricity, frequent brushing.
• Drum filter: high fineness and automatic cleaning, the benchmark for gravity-fed systems.
• Roller filter: very fine, but with a consumables cost.
• The right prefilter depends on the pond, budget and available maintenance time.

Last updated: October 3, 2026.