Which pump and filter for my pond? (table)

In short

• First, calculate the volume: length × width × average depth = m³.
• General rule: pass the entire volume through the filter approximately every 2 hours without fish, and at least once an hour with koi.
• The flow rate that matters is the actual flow rate at the filter, not the maximum flow rate stated on the pump.
• Choose the filter based on the capacity recommended by the manufacturer for your population (fish or koi). Never undersize it.

To choose a pond pump and filter, two pieces of information are enough to get started: the water volume in m³ and the type of fish. The volume determines the flow rate that needs to circulate. The population determines the required filtration capacity. A koi pond requires significantly more than a planted pond with a few goldfish. This guide gives you simple guidelines, a flow rate table, and a step-by-step method.

Step 1: calculate the volume of your pond

Multiply the length by the width, then by the average depth. Do not use the maximum depth: you would overestimate the volume. For a freeform shape, divide the pond into rectangles and add the volumes.

Example: a pond measuring 4 m × 3 m with an average depth of 0.8 m contains 4 × 3 × 0.8 = 9.6 m³, or approximately 9,600 litres.

Be careful not to confuse m² and m³. A surface area of 12 m² does not correspond to a volume of 12 m³.

Step 2: identify the fish population

Fish produce waste that the filter must treat. The more they are, the larger and the more they are fed, the higher the load. Koi grow quickly and eat a lot: a koi pond is always sized separately.

• Planted pond, without fish or with very few small fish.
• Goldfish pond or small ornamental fish, in reasonable numbers.
• Koi pond, or very heavily stocked pond, or heavily fed fish.

Step 3: determine the recommended flow rate

The rules below are common and cautious guidelines, not absolute values. They indicate how many times the pond volume should pass through the filter per hour.

• Planted pond: approximately 0.5 times the volume per hour (the entire pond filtered in 2 hours).
• Goldfish: from 0.5 to 1 times the volume per hour, depending on the density.
• Koi: at least 1 times the volume per hour.

Table of recommended actual flow rates at the filter

These values are calculated based on the guidelines above. For a very sunny, heavily stocked pond, or one where the fish are growing, aim for the top of the range.

Step 4: choose the pump based on its actual flow rate

The flow rate displayed on a pump is a maximum flow rate, measured without height or pipe length. In your installation, the flow rate decreases with:

• the height between the water surface and the filter inlet or the top of the waterfall;
• the length of the pipe and the number of elbows;
• a pipe diameter that is too small;
• the clogging of the strainer.

Consult the pump's flow rate curve on its data sheet. Read the flow rate at the actual height of your installation. This is the figure that must reach the value in the table. Preferably choose a low-consumption filtration pump, designed to run continuously.

Discover our pond pumps for filters and streams.

All models are also grouped in our pond pumps section. For details on the criteria (consumption, height, pump type), also read how to choose a pump for your pond.

Step 5: choose the filter based on the right capacity line

Manufacturers often indicate several capacities for the same filter: without fish, with fish, with koi. Always take the line that corresponds to your pond. A filter advertised "up to 20 m³" without fish may be suitable for a much smaller volume with koi.

Our rule: never undersize the filter. If in doubt between two models, take the larger one. A filter that is too small will clog quickly, require more maintenance, and allow the water quality to deteriorate.

What type of filter?

• Pressure filter: easily hidden, can return the water to a waterfall. Suitable for small and medium ponds.
• Gravity or multi-chamber filter: large biological capacity, easy maintenance. It must be placed higher than the pond, or fed by gravity.
• Complete kit: pump, filter, and UV already matched with each other. Practical for a first pond.

See our garden pond filters, our multi-chamber filters, and our complete filtration kits.

Complete OASE kits, ready to install

Here are four kits from our section, from the small planted pond to the large koi pond. Always read the capacity line that corresponds to your population.

Capacities indicated on our product sheets, according to OASE data. Foudebassin recommends leaving a margin, especially with koi.

Step 6: check the compatibility of the pump, filter, and UV

1. Check the maximum flow rate accepted by the filter. A gravity filter fed by a pump that is too strong can overflow.
2. Check the flow rate range of the UV-C sterilizer. A flow rate that is too high reduces its contact time with the water.
3. Check the connection diameters: pipe, filter inlet, pump outlet.
4. Place the pump elevated, out of the sludge, at the opposite end of the water return to circulate the entire pond.

For the UV, see our UV-C devices for ponds, to be chosen according to the manufacturer's indications for your volume and your population.

For the power and proper placement of the UV, also see how to choose the right UV for your pond.

Complete example

Pond of 9.6 m³ with 6 koi. Koi guideline: at least 1 times the volume per hour, or approximately 10,000 l/h actual at the filter. The filter is 1 m above the water, with 8 m of pipe. We choose a pump whose curve gives at least 10,000 l/h at this height. The filter must be advertised for at least 10 m³ with koi, and accept this flow rate. With goldfish instead, an actual flow rate of 5,000 to 10,000 l/h would suffice.

The most common mistakes

• Calculate the volume with the maximum depth.
• Choose the pump based on its maximum flow rate.
• Read the "without fish" capacity of the filter for a koi pond.
• Plan for a filter that is just sufficient, without a margin for the growth of the fish.
• Place the pump directly on the bottom, in the sludge.

To go further: the five questions to ask yourself before choosing a filter.

Your pre-ordering checklist

Keep this list handy when comparing product sheets. If one line remains unanswered, our team can complete it with you.

• The volume is calculated with the average depth, rounded up.
• The population is identified: planted pond, goldfish, or koi.
• The target flow rate comes from the table, according to the population.
• The height between the water and the filter (or the top of the waterfall) is measured.
• The flow rate of the pump is read on its curve, at this height.
• The filter is chosen based on its "with fish" or "with koi" line, with a margin.
• The filter and the UV-C accept the flow rate of the pump.
• The pipe and fitting diameters are compatible.

Maintain the flow rate throughout the season

Well-sized equipment loses its efficiency if it is not maintained. The actual flow rate drops as soon as the strainer, pump, or filter sponges get dirty. A few simple steps are enough to maintain it.

1. Check the pump strainer regularly during the season, and remove leaves and debris.
2. Clean the filter according to the manufacturer's instructions, using the provided functions (cleaning handles, bottom valve, rinsing). Rinse the sponges with pond water rather than tap water, to preserve the useful bacteria.
3. Descale the pump when the flow rate drops while the strainer is clean.
4. Check the UV-C: clean quartz and lamp replaced according to the manufacturer's instructions.

Well-sized filtration helps the water stay stable, but it says nothing about its composition. Water that turns green or cloudy is often the result of an imbalance: excess nutrients, organic matter, light, or lack of oxygen. Before adding a product, check with our free online pond water analysis.

Need advice for your pond?

Send us the volume (length, width, average depth), the number and type of fish, and the height between the water and the filter. We will help you choose a suitable pump and filter.

Phone: +32 10 86 03 54 – Email: support@foudebassin.com. Response in business days.

Frequently asked questions

Is a more powerful pump always better?

No. A pump that is too strong can cause a gravity filter to overflow and reduce the efficiency of the UV. The right choice is an actual flow rate close to the guideline, compatible with the filter and the UV.

Should the pump run day and night?

Yes, during the season. The bacteria in the filter need a continuous supply of water. A prolonged shutdown deprives them of oxygen.

My pond is freeform, how do I calculate the volume?

Divide it into rectangular areas, calculate each volume with its average depth, then add them up. The result remains an estimate: round up.

I'm adding koi to a goldfish pond: do I need to change the filter?

Often, yes. Check the "koi" capacity of your current filter. If it is lower than your volume, you need a larger filter or a second filter. Contact us with the model number of your equipment.

What pump flow rate for a 10 m³ pond?

Allow for an actual flow rate to the filter of around 5,000 l/h for a planted pond, 5,000 to 10,000 l/h with goldfish depending on the density, and at least 10,000 l/h with koi. Read this flow rate on the pump curve, at the actual height of your installation.

Is a complete kit a good choice for a first pond?

Yes, it is often the easiest: the pump, the filter and the UV-C are already matched with each other. Choose the kit based on its "with fish" or "with koi" capacity, and leave some margin, especially with koi.

How to clean a limescale-covered pond pump?

Never use bleach. Clean the strainer, then descale the pump with a product intended for this use, such as OASE AquaActiv PumpClean, following its instructions for use.