Constructed wetland for a fish pond: diagram, sizing, and plants

A constructed wetland is that strip of gravel planted with rushes and irises that borders some ponds and seems to work miracles: crystal-clear water, few algae, and almost no visible technology. The principle is appealing, but it was initially designed for lightly loaded water. As soon as there are fish, and especially koi fed every day, the rules change. A well-designed constructed wetland can handle a large part of the purification of a fish pond, but never without prefiltration and rarely on its own. Here's how to size, build, and plant it while taking your pond's actual load into account.

How constructed wetlands work

A pond constructed wetland is a planted filter. Water flows slowly through a bed of gravel inhabited by billions of bacteria attached in a biofilm, in contact with the roots. Several mechanisms work simultaneously:

• nitrification: bacteria convert the ammonia excreted by fish into nitrites and then nitrates, which are much less toxic;
• denitrification: in oxygen-poor areas of the bed, other bacteria convert some of the nitrates into nitrogen gas;
• absorption by plants, which take up nitrates and phosphates for their growth and are removed when the plants are trimmed;
• the physical retention of fine particles and the fixation of some phosphorus onto certain substrates.

Contrary to popular belief, plants are not primarily responsible for purification: they oxygenate the root zone, host bacteria, and export nutrients, but most of the work is done by bacteria. Limnology textbooks, from Wetzel to Scheffer, clearly demonstrate this plant-bacteria partnership in natural wetlands. A garden constructed wetland is a concentrated version of it.

Why a fish pond is more demanding

A natural swimming pool receives almost no input: a little dust, a few leaves, and bathers. A fish pond receives food every day, and therefore nitrogen and phosphorus, as well as excrement. Boyd and Tucker explain this for fish-farming ponds: most of what enters as food ends up in the water as dissolved waste or suspended matter.

Three practical consequences:

1. The solids load is high. Feces, uneaten food, and dead algae clog everything they pass through. An unprotected bed of gravel becomes clogged.
2. Oxygen demand is high, especially at night and in summer. A poorly supplied planted filtration zone can become an anoxic area that returns oxygen-poor water.
3. The load varies with the seasons: high in summer when feeding is heavy, low in winter when plants are dormant. Planted filtration alone struggles to keep up with these variations, hence the value of combining it with a technical filter.

The more you feed, the larger the required planted filtration area: the amount of food distributed is the best indicator of the load, far more than the pond volume.

Planted filtration area according to the load

There is no universal formula, and beware of overly precise figures. The guidelines below are rough estimates based on field experience, to be adapted to your sunlight, your plants, and above all the amount of food.

For a koi pond, the logic is therefore reversed: the technical filtration is sized as if there were no planted filtration zone, and the latter is added to further refine the water and limit nitrates and algae. It is a safety measure, not a replacement.

Structure: gravel, depth, flow

Ideally, the constructed wetland should be built in a separate tank from the main pond, with its own EPDM liner laid over underlay. This prevents the fish from disturbing the gravel and the roots from spreading into the pond.

The key is to force the water to pass evenly through the filter bed. Water that flows across the surface or follows a preferential path purifies almost nothing. Two layouts are possible:

• Upflow: pumped water is injected at the bottom, beneath the gravel, through a network of perforated pipes. It rises through the filter bed and overflows into the pond. This is the most common setup for fish ponds, and is simple to supply with a pump placed in the pond or after the prefilter.
• Downflow: water is distributed across the surface and filters from top to bottom down to bottom drains, from which a pump draws it. It oxygenates the filter bed better but requires more careful design.

The advantages and disadvantages of each are detailed in our article on downflow or upflow constructed wetlands, linked below. For a first project involving a fish pond, an upflow system supplied with well-prefiltered water remains the most forgiving.

Feeding and prefiltration

This is the part most often neglected, and the number one cause of constructed wetlands that "stop working" after two or three years. Unretained particles accumulate between the gravel, form organic sludge, and eventually the water can flow through only a few channels.

The basic layout of a fish pond with a constructed wetland:

1. Water intake at the lowest point of the pond and through a surface skimmer.
2. Mechanical prefiltration: foam filter, brush chamber, sieve, or vortex, depending on the pond size. Its role is to retain solids before they reach the gravel.
3. Pump positioned after the prefilter or in a pumping chamber.
4. Constructed wetland, optionally preceded by a biological filter or UV unit depending on the load.
5. Gravity-fed return to the pond through a sheet of water or a small stream, which reoxygenates the water as it flows.

In terms of flow rate, a slow, continuous flow is preferable to a strong intermittent flow: bacteria need a regular supply of oxygenated water, and a pump that stops for several hours allows the bed to become anoxic. For a medium-sized pond, renewing the volume over a few hours through the entire filtration and lagoon system is a common rule of thumb; the prefilter should handle the flow, not the gravel. If you prefer a simplified approach without a dedicated pump, our guide how to make a natural pond filter presents more modest alternatives.

Suitable water-purifying plants

In a lagoon filter for a fish pond, look for fast-growing plants with dense, fibrous roots that can tolerate having their feet permanently in damp, nutrient-poor gravel. It is better to mix several species: each has its own growth rate and preferences, and the system is more resistant to disease and winter conditions.

Absolutely avoid species listed on the European list of invasive alien species (Regulation 1143/2014), such as water primroses, floating pennywort, or parrot feather: their sale and planting are prohibited, and they can escape into waterways. To broaden your options, see plants for lagooning and biofiltration and our general guide which plants for a pond.

Plant directly in the gravel, using bare roots or washed root balls: potting soil would add nutrients and fines that cause clogging. A density of a few plants per square meter is sufficient; the planting bed will fill in within one or two seasons.

Annual maintenance

A constructed wetland requires little work, but that work must be regular. A typical schedule looks like this:

• Late winter or early spring: cut back the dry parts before growth resumes, removing them from the area to export nutrients. Check water distribution (dry areas and areas where water stagnates at the surface).
• Spring and summer: clean the prefilter as often as necessary; it is what protects the gravel. Thin out overly invasive plants.
• Autumn: prevent dead leaves from accumulating on the gravel and reduce fish feeding.
• Every few years: check for clogging. If water no longer passes through the planting bed, the drains must be rinsed, and some of the gravel may even need to be removed and washed.

Adding bacteria when restarting in spring can speed up recolonization of the planting bed, but it can never replace effective prefiltration or a continuous food supply.

Combining a constructed wetland and a mechanical filter

The most reliable configuration for a fish pond combines both approaches. The technical filter handles load peaks, retains solids, and ensures nitrification even in spring, when the plants are still dormant. The constructed wetland takes over for refinement: it consumes nitrates and phosphates, competes with algae, and provides the crystal-clear, slightly amber-colored water typical of natural ponds.

A common case: a koi pond whose water remains slightly green despite a good filter. Adding a finishing constructed wetland, even a modest one, often changes the situation within a season because it targets the nutrients that the filter lets through. Conversely, a constructed wetland alone on an overcrowded pond will only accumulate sludge.

Also read

Choose the flow direction of your planted filter: Downflow or upflow planted filter.

READ THE ARTICLE

Frequently asked questions

— Can a planted filter replace the filter for a koi pond?

Rarely. For koi that are fed regularly, the planted filter complements mechanical and biological filtration sized for the pond. It polishes the water but cannot handle load spikes on its own.

— How large should a planted filter be for a fish pond?

As a general guide, 20 to 30% of the pond's surface area for a lightly stocked pond, and 30 to 50% for a well-stocked pond. The more you feed, the more surface area or technical filtration you need.

— Should soil be added to a planted filter?

No. Washed gravel or lava rock is used, without fines or soil, so that water can circulate and plants can draw nutrients from the pond water.

— Why does my planted filter smell bad or no longer filter?

The planted bed is probably clogged with sludge due to a lack of prefiltration, or the flow is too weak or intermittent. Clean the prefilter, check the water distribution, and rinse the drains.

In summary

• The planted filter purifies water mainly through bacteria in the bed; plants remove nutrients.
• A fish pond has a high organic load: mechanical prefiltration is mandatory.
• Indicative surface area: 20 to 50% depending on the load, as a complement to a filter for koi.
• Separate tank, 30 to 60 cm of washed gravel, forced upflow or downflow circulation.
• Vigorous, non-invasive plants, annual pruning, and a clean prefilter.

Last updated: October 3, 2026.