Building a fish pond: volume, depth, filtration, and protection - the choices that matter

At first glance, an ornamental pond and a fish pond look alike, but they do not meet the same requirements. The former relies mainly on its plants; the latter must also handle the daily waste of growing animals, breathe during heatwaves, remain ice-free in January and withstand visits from herons. Creating a fish pond first means deciding on the volume, depth and filtration according to the fish you want to put in it, even before taking the first shovel. The digging and liner installation come next and follow the same steps as for any pond. It is the design choices made on paper that determine whether the fish will live well there for ten or twenty years.

What changes when the pond is designed for fish

A fish-free, well-planted pond often finds its own balance: plants consume nutrients, microorganisms decompose fallen leaves, and local wildlife settles in on its own. As soon as fish are added, they introduce a continuous source of waste. Every meal ends up as ammonia excreted through the gills and droppings that settle on the bottom. The larger and more numerous the fish, the greater this flow, and the more actively the water must be treated.

In concrete terms, a fish pond is defined by four requirements:

• sufficient volume to dilute waste and buffer temperature fluctuations;
• a deep area that stays cool in summer and frost-free in winter;
• circulation and filtration capable of converting ammonia into less toxic compounds;
• protection against predators, especially herons.

The rest, namely the shape, stones, plants and waterfall, is a matter of aesthetics, provided it does not conflict with these four points.

Choose the fish before choosing the pond

The question we often ask too late is usually the first one to ask: which fish do you want to see in ten years? A goldfish bought at a few centimetres can exceed twenty centimetres, an orfe can easily reach thirty to forty, and an adult koi commonly exceeds sixty centimetres. The pond should be sized for the adult size, not the size at purchase. Our article on fish suitable for an outdoor pond reviews the main species.

The table below gives rough figures commonly recommended by aquatic gardening specialists for a climate like Belgium's. These are not regulatory thresholds, but sensible guidelines: below them, it may be possible, but the balance becomes fragile and requires much more attention.

Sturgeons, sometimes recommended for ponds, are a special case: they grow very large, need highly oxygenated water and large areas of clear bottom, and do not tolerate warm water well. They are not suitable for a first pond. As for tropical aquarium fish, they obviously have no place outdoors in our latitudes.

For the number of fish, the golden rule is moderation: a sparsely stocked pond stays clear and stable, while an overstocked pond requires very powerful filtration and cannot tolerate any breakdown. Density guidelines are detailed in our article on the appropriate number of fish for a pond.

Depth and shape: think about winter, summer, and herons

A pit that does not freeze

In winter, water reaches its maximum density at around 4 °C and accumulates at the bottom, while ice forms on the surface. This stratification, well described in limnology, allows fish to spend the winter in a state of reduced activity in the deep layer. The pond must still be deep enough to prevent the ice from reaching them. In our latitudes, a zone at least 80 cm to 1 m deep is the minimum for goldfish, while koi fare considerably better at approximately 1.2 to 1.5 m.

A cool zone in summer

The same deep volume plays the opposite role in summer: it warms up less quickly than shallow shelves and provides a refuge when the surface exceeds comfortable temperatures. Warm water contains less dissolved oxygen, while fish, being more active, consume more of it. A shallow pond in full sun suffers from both disadvantages.

Walls that discourage herons

Herons fish on foot, wading through shallow water. A fish pond therefore benefits from clear-cut edges and a deep swimming area, accessible without crossing a long stretch of shallow water. Plant shelves are still useful, but they should be limited to part of the perimeter rather than extending all the way around. For koi, many ponds are built with almost vertical walls and a single planted area, or even a separate planted filtration area.

A shape that lets water circulate

Narrow peninsulas, coves, and corners may look attractive on a plan, but water stagnates there, waste accumulates, and oxygen levels drop. A compact, oval, or broad kidney-shaped layout allows the pump to keep the entire volume moving. If you want a complex shape, plan for a second intake or a water return that sweeps across the isolated area.

Filtration: size it for the fish, not just the volume

The filter has two functions. Mechanical filtration retains particles (droppings, plant debris, suspended algae) to keep the water clear. Biological filtration houses bacteria that transform toxic ammonia into nitrites, then into much better-tolerated nitrates. An UVC clarifier often completes the setup by flocculating the microscopic algae responsible for green water.

Manufacturers indicate a maximum pond volume for each filter, but this figure drops significantly when the pond is heavily stocked, very sunny, or inhabited by koi. The usual practice is to divide the stated capacity by two for a koi pond and choose the next larger model if in doubt. An oversized filter has no drawbacks for fish; an undersized filter results in cloudy water and nitrite spikes.

Pump flow rate

As a general rule, aim to circulate the entire pond volume in one to two hours for goldfish, and preferably in one hour or less for koi. However, the pump must remain within the flow rate range accepted by the filter and UVC: too much flow, and the water passes through the filter without being treated. Also consider the pumping height and hose length, which reduce the actual flow rate compared with the stated flow rate.

Gravity-fed or pressure filter?

The gravity-fed filter is installed above water level; the pump sends water to it, and the water flows back down into the pond by gravity, often as a waterfall. It is easy to maintain and highly effective biologically, but it must be concealed. The pressure filter can be partially buried and return water higher or farther away; it is discreet but slightly less accessible. Our guide to choosing your pond filter compares the two types.

Bottom intake, return on the opposite side

The pump is placed at the deepest point, where waste settles, and the water return on the opposite side, so the current flows through the entire pond. For koi ponds, a bottom drain connected to the filter is the most effective solution: it continuously removes sediment without a submerged pump in the pond. It must be planned when digging the pond, never added afterward. See our article on the importance and installation of a bottom drain.

Oxygen: the secret to summer nights

Fish deaths in ponds often occur at dawn, after a hot and stormy night. The reason is simple: at night, plants and algae no longer produce oxygen but consume it, as do fish and bacteria. In muggy weather, exchange with the air is limited. In the early morning, dissolved oxygen reaches its lowest level, and the fish come to "pipe" at the surface.

A waterfall or water return that disturbs the surface helps, but an air pump with one or more diffusers placed in the deep area is the best safeguard, especially in a densely stocked pond. Leave it running continuously from June to September; in winter, place the diffuser halfway up instead, so as not to stir the deeper, warmer layer where the fish stay.

Plants: allies of clear water, to be protected from fish

Plants consume the nitrates and phosphates that would otherwise feed algae, and they provide shade. As a rough guide, aim to have one third to one half of the surface covered by floating leaves or shaded by bank vegetation, plus a few submerged plants. Research on shallow lakes, notably synthesized by Marten Scheffer, clearly demonstrates the role of vegetation in maintaining clear water.

However, be aware that goldfish nibble, while koi pull up and uproot plants. In a koi pond, plants survive better in baskets weighted with pebbles, on shelves protected by a grid, or in a separate wetland zone through which the filter water passes before returning. Use poor soil or a specific potting mix for aquatic plants, never enriched garden soil.

Several plants once commonly available in the trade, such as Brazilian watermilfoil, parrot's feather and lagarosiphon, are now banned in the European Union under the regulation on invasive alien species. Choose native species or authorized ornamental varieties.

Protecting fish from predators

The grey heron is the main predator of garden ponds. It is a protected species: the aim is to deter it, never to harm it. Effective solutions combine several obstacles:

• an accessible deep area where the fish take refuge as soon as a shadow passes;
• submerged shelters: large ceramic or PVC tubes placed on the bottom, overhanging stones, water lily leaves;
• edges without beaches around most of the perimeter;
• a taut net above the water in autumn and late winter, when visits are most frequent;
• a taut line about twenty centimetres above the ground and bank, which makes approaching on foot difficult.

Plastic decoys rarely work for long: herons quickly get used to them if they do not move. Cats and, more rarely, kingfishers or otters add to the list of visitors, but a deep pond with sheer edges discourages most of them.

Filling the pond and maturing the filter

A new pond is biologically empty. Nitrifying bacteria, which convert ammonia into nitrites and then nitrates, must colonize the filter media and surfaces before they can process fish waste. This is the nitrogen cycle, described in all quality water-management manuals for fish farming. Until it is established, ammonia and especially nitrites can reach toxic levels, even with very few fish.

The sensible approach:

1. fill the pond, start the pump and filter, and run them continuously without switching them off at night;
2. plant as soon as the pond is filled;
3. seed the filter with activation bacteria, which speeds up the start-up without making it instantaneous;
4. test the water once or twice a week: the nitrites rise, then fall again;
5. introduce the first fish when the nitrites have returned to zero, in small numbers.

Allow roughly three to six weeks depending on the temperature: the cycle starts slowly in cold water in early spring. Our article on how long to wait before adding fish to a new pond explains this schedule in detail.

Gradual stocking

Even with a mature filter, bacteria only multiply according to the food available. Introducing all the planned fish at once causes another nitrite spike. It is better to proceed in waves, a few weeks apart, testing the water after each introduction.

• Acclimate the fish by letting the bag float in the shade for about twenty minutes, then gradually adding a little pond water to it.
• Avoid pouring the bag water into the pond: it may contain parasites or germs from the place of sale.
• Quarantine newcomers in a separate tank when the pond already houses valuable fish, especially koi: this is the best protection against introducing diseases, as fish pathology textbooks point out.
• Feed sparingly during the first few weeks: anything not eaten within a few minutes pollutes the water.

Finally, pond fish should never be released into a pond, river, or ditch. Released goldfish can proliferate and disrupt natural habitats. If a pond needs to be dismantled, give the fish to another owner or an aquatic garden center.

The annual cycle in a fish pond

A well-designed pond requires little intervention, but tasks need to be done at specific times. The following table summarizes the essentials.

The most common design mistakes

• A pond sized for 5 cm fish that becomes too small in three seasons.
• A maximum depth of 50 to 60 cm, too shallow for both winter and summer.
• A filter chosen barely large enough based on the volume stated by the manufacturer, without taking the fish into account.
• A pump placed near the water return, leaving half the pond without water circulation.
• No power supply provided for an air pump.
• Shallow areas all around, ideal for herons.
• Fish introduced on the day the pond is filled.

All these mistakes are difficult to correct afterward, because they concern the pond itself or the pipes. That is why a plan, even a rough sketch, showing the depths, the location of the pump, the filter, and the water return is better than spending several hours redoing earthworks.

Also read

Before sizing your pond: Which fish for my outdoor pond?.

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Frequently asked questions

— What is the minimum volume for a goldfish pond?

As a general rule, a few cubic meters are enough for a small group of goldfish, with an area at least 80 cm to 1 m deep in a climate like Belgium's. The greater the volume, the more stable the water.

— Can you have a fish pond without a filter?

For a few small fish in a large, heavily planted pond, it is possible. As soon as there are many fish, large fish or daily feeding, a pump and biological filter become essential for removing ammonia and nitrites.

— How can fish be protected from herons?

Combine a deep area, submerged shelters, clear edges without a shallow beach and, during high-risk periods, a net or wire stretched along the edge. The heron is protected: deter it without ever harming it.

— Is an air pump necessary in a fish pond?

It is strongly recommended, especially in summer and in populated ponds: warm water contains less oxygen, and levels are lowest in the early morning. A diffuser in a deep area helps keep fish safe during hot nights.

In summary

• Choose the fish first and size the pond for their adult size.
• Plan for a deep, frost-free area that stays cool in summer, and a shape without dead corners.
• Pump at the lowest point, return on the opposite side, filter sized for the fish load, aeration for summer.
• Clear edges, shelters and a heron net, plants protected from koi, non-invasive species.
• Let the filter mature for a few weeks and stock the pond in stages while monitoring nitrite levels.

Last updated: October 3, 2026.