Build a multichamber pond filter: plans, materials, and budget

A row of tanks, a few tank connectors, brushes, a mat and biological media: a homemade multichamber filter can provide much more filtration volume than an off-the-shelf model at the same price. The key is to make what is simple - the tanks and plumbing - and buy what is technical: the pump, UV-C, aeration and media. Here is a modular plan, an itemized costed shopping list and the mistakes to avoid.

Should you build or buy?

A multichamber filter is nothing mysterious: a few tanks connected to one another, through which water successively passes through increasingly fine media and then biological media. With a little equipment and method, a DIY enthusiast can build one that rivals many commercial models. But homemade is not always the best deal.

DIY makes sense when:

• you need a large filtration volume for a koi pond, and commercial filters of this size exceed your budget;
• you have sound reclaimed tanks and space to install them;
• you enjoy DIY and are willing to spend a few weekends on the project;
• you want a modular installation that you can expand chamber by chamber.

It's better to buy when:

• the pond is small: a well-chosen off-the-shelf filter costs less than a DIY installation and takes up less space;
• you want an automatic prefilter like a drum filter: its mechanics, electronics and screen cannot reasonably be made at home;
• appearance matters and the filter will be visible;
• you don't have time to adjust and fine-tune things during the first few weeks.

The best compromise is often to build the tanks and plumbing, and buy all the technical equipment: pump, UV-C, air pump, media. The plan that follows is based on this principle.

Choosing the tanks (drums, IBCs, chests)

Three main types of containers are used for homemade filters:

Whatever the tank, three rules are non-negotiable:

1. Reject any tank of uncertain origin. A tank that has contained chemicals, hydrocarbons, or pesticides must never be used for a pond, even after washing. Prefer tanks that have contained food products, and rinse them thoroughly.
2. Protect from light. White or translucent tanks let light through: algae develop on the walls and the plastic deteriorates in the sun. Paint them, clad them with siding, or install them in a shelter.
3. Check the rigidity. A full tank weighs as many kilograms as its volume in litres. A wall cut out for a wall fitting must not deform, otherwise the seal will leak. On an IBC, the metal cage is often kept for this reason.

Designing the chambers: settling, mechanical, biological

An effective multichamber filter follows the same logic as commercial models: separate the stages. Here is a modular plan for a koi pond of around fifteen cubic metres, in a pumped system, with three tanks of approximately 200 to 300 litres or a partitioned IBC.

The principle of alternating inlets and outlets, once at the top and once at the bottom, forces the water to pass through the full height of each chamber instead of flowing across the surface. In the settling chamber, the inlet is ideally positioned low down and oriented to create a slight rotation that encourages sediment to settle in the center.

The total filtration volume is determined more by the fish load than by the pond volume. For a koi pond, many owners aim for a filter volume of around 10% of the pond volume, providing comfortable chambers and less frequent cleaning. This is a general guideline, not an absolute rule.

Plumbing: wall fittings, diameters, overflow

The plumbing is the most technical part of a homemade filter, and where most leaks occur.

• Wall fittings pass through the tank wall and are secured with a nut and seals on both sides. Choose quality screw-in models suited to the pipe diameter, and drill with a hole saw exactly matching the specified diameter. The wall must be flat at the drilling point: on a rounded tank, a wall fitting installed on a curve will almost always leak.
• The diameter depends on the flow rate. Between the chambers, the water flows by gravity, with very little difference in water level. For a flow rate of around 8 to 10 m³/h, a 110 mm passage is the minimum; above that, use two passages. A diameter that is too small causes the water level in the first chamber to rise until it overflows.
• The return to the pond also works by gravity, preferably through a 110 mm pipe, with a consistent slope.
• The safety overflow is essential. If a grille or foam becomes clogged, the water must overflow toward the pond, not into the garden, or the pond could be emptied within a few hours. Therefore, an overflow connected to the return is installed at the top of the first chamber.
• Drains: a valve at the lowest point of each chamber, connected to a drain, allows sludge to be removed without dismantling the filter.

For the pipes, use glued pressure-rated PVC, with suitable adhesive and a primer cleaner. Allow it to dry according to the adhesive manufacturer's instructions before filling the system with water.

Chamber filling

The filling follows the logical order of any filtration system: from coarsest to finest for the mechanical stage, followed by biological filtration.

1. Chamber 1: filtration brushes suspended from a grate or wooden battens, packed tightly enough to touch, across the entire width. Leave open space at the bottom for sludge to settle and be drained.
2. Chamber 2: vertical Japanese mat sheets, perpendicular to the direction of water flow, covering the entire cross-section; or foams with progressively finer grain, from coarsest to finest. Leave no gaps at the sides through which water could bypass the media.
3. Chamber 3: biological media. In a moving bed, fill approximately 50 to 60% of the chamber's water volume and provide aeration from the bottom. In a static bed, use ceramic or plastic rings in mesh bags, placed on a false bottom.

The choice and quantities of each medium are detailed in our guide to choosing the right filter. One rule to remember: never place ceramic or fine media in the first chamber, where it would become a sludge trap.

Integrating UV and aeration

Two pieces of equipment complement a homemade filter, and these are precisely the ones that are best bought ready-made.

UV-C is most often installed between the pump and the filter, on the inlet pipe. It destroys the single-celled algae that cause green water, which are then trapped by the brushes and mat. Its sizing depends on the pond volume, sunlight exposure, and flow rate passing through it: excessive flow reduces the exposure time. Our article on choosing the right UV explains this calculation in detail. Remember that the lamp loses efficiency over time and is generally replaced each season.

Aeration is essential if the biological chamber operates as a moving bed. A diaphragm air pump, installed in a dry, sheltered location, supplies one or more air diffusers placed at the bottom of the chamber. Even in a static bed, gentle bubbling improves bacterial oxygenation. Install a non-return valve if the air pump is lower than the water level.

Budget and comparison with a commercial filter

Here is a typical shopping list for the three-chamber setup described above. Prices are indicative estimates that vary depending on brands, promotions, and the condition of used tanks.

Overall, the total most often comes to between €1,000 and €2,500, with about half going toward the pump, UV-C, and aeration, which you would have to buy anyway with a commercially made filter. A standard multichamber filter sized for around fifteen cubic meters with koi, on the other hand, often costs more than €1,500 without the pump or UV.

The real savings from building it yourself therefore mainly concern the tanks and the filtration volume: for the same budget, you get a larger filter. In return, you have to allow for construction time and adjustments, and accept a rougher appearance, which can be improved with wooden cladding.

Common case: a homeowner obtains two food-grade IBCs and builds a 2,000-liter filter for a 20 m³ pond. The filter works well, but the first chamber regularly overflows in summer. The cause: a single 110 mm passage between the chambers for a flow rate of more than 12 m³/h. Adding a second wall fitting solves the problem. This is the most common DIY mistake: undersizing the water passages.

Start-up and initial adjustments

Before connecting the filter to the pond, perform a clear-water test: fill the tanks, start the pump on a closed circuit if possible, and monitor each wall fitting for several hours. A leak is much easier to spot on a new, clean filter than once the media is in place.

Next:

1. Adjust the flow so that the level in the first chamber remains stable, a few centimeters below the overflow.
2. Check the circulation in the biological chamber if it is a moving bed: the entire mass should be turning.
3. Seed with starter bacteria and, if possible, a little colonized media from an existing filter.
4. Feed sparingly during the first few weeks and measure nitrites twice a week.
5. Leave the UV-C off for a few days after seeding, then turn it back on.
6. Plan maintenance: drain the settling chamber every week during the season, rinse the brushes and mat when they become loaded, never all the chambers on the same day.

A well-designed homemade filter can operate for many years. Its strength lies in its modularity: an additional chamber, a more efficient prefilter or a larger moving bed can be added as the pond evolves.

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Frequently asked questions

— Which tank should be used for a homemade pond filter?

Rigid tanks that previously held food products, protected from light. 1,000-liter IBCs are widely used; never use a tank that previously held chemicals.

— What diameter should there be between the chambers of a homemade filter?

For a flow rate of around 8 to 10 m³/h, at least one 110 mm passage per partition; above that, double the passages to prevent the first chamber from overflowing.

— How much does a homemade multichamber filter cost?

Usually between €1,000 and €2,500 all-inclusive, with a large share going toward the pump, UV-C and aeration. The tanks and plumbing are the most economical part.

— Is an overflow needed on a homemade filter?

Yes. In the event of clogging, the water must overflow toward the pond and not into the garden; otherwise, the pond may empty within a few hours.

In summary

• Build the tanks and plumbing, buy the pump, UV-C, aeration system and media.
• Rigid, food-grade tanks protected from light.
• Three chambers: settling and brushes, fine mechanical filtration, aerated biological filtration.
• Sufficient 110 mm wall penetrations, overflow toward the pond, drains at the lowest point.
• Usually between €1,000 and €2,500 all-inclusive.
• Leak test with clear water, seeding and gradual increase in load.

Last updated: October 3, 2026.