What fish should I put in a 4,500-liter pond?

The question comes up constantly on forums: “I have a pond of around 4,500 litres; what can I put in it?” The answers often range from “three koi will be fine” to “nothing at all without a filter.” A volume of 4.5 m³ is a good size for goldfish, shubunkins, comets, or a few small native species, provided you consider the fish's adult size rather than their size when purchased. Here's how to decide, species by species.

What exactly is 4,500 litres?

Before talking about fish, here's a reminder that avoids a lot of disappointment: the stated volume is rarely the actual volume. A pond described as “4,500 litres,” calculated when it is dug, often loses 15 to 25% once the plants, baskets, stones, and bank area have been installed. And what matters to fish is not just the volume, but also the surface area exposed to the air and the depth of the deepest area.

In practical terms, a 4.5 m³ pond most often looks like a basin measuring about 3 × 2.5 m with a pit 80 cm to 1 m deep. This is a comfortable size for a garden, large enough to keep the water stable if properly equipped, but it remains a small volume for a fish that can live for twenty years.

In our Belgian climate, the recommended minimum depth for fish to overwinter outdoors is around 80 cm in part of the pond. Below that, the water cools too quickly in winter and warms too much in summer.

Species that are suitable, and those to forget

Koi require a specific clarification. You can certainly start with three 12 cm koi in 4,500 liters, and everything will be fine for two or three seasons. The problem arises when they reach 40 or 50 cm: the organic load increases dramatically, the water becomes unstable, and the fish no longer grow properly. A pond should be sized for the fish it will contain in five years, not for the ones in the transport bag.

Ide are beautiful at the surface, but they live in schools and swim quickly. In 4.5 m³, they swim in circles and easily jump out of the water. If you really want them, keep three or four at most, in an area of at least 7 to 8 m², and no other large fish.

How many fish, specifically?

There is no universal rule, because everything depends on the filtration, plants, sunlight exposure, and amount of food provided. In practice, for a well-filtered and properly planted 4,500-liter pond, you can use these rough guidelines:

• without a filter, with plenty of plants: 5 to 8 adult goldfish, fed sparingly;
• with suitable biological filtration: 10 to 15 adult goldfish or shubunkins;
• with minnows: a group of around fifteen, in addition to a few goldfish.

The classic trap is reproduction. Goldfish readily spawn as soon as the water warms up in spring, and a well-planted pond allows some of the fry to survive. After two or three years, the original ten fish can become forty. Plan a solution from the outset: give fish to other enthusiasts or limit spawning areas. Releasing fish into the wild, however, is strictly prohibited.

The equipment that makes this volume comfortable

In a small volume, the margin for error is small. Too much food, a hot day, or a pump failure quickly becomes noticeable. Three factors make the difference.

Biological filtration sized with capacity to spare. Manufacturers often state a maximum volume without fish and a significantly lower volume with fish. For a stocked 4.5 m³ pond, choose a filter whose “with fish” capacity clearly exceeds your actual volume, checking these figures on the manufacturer's technical data sheet.

Supplementary aeration. Warm water contains less oxygen, and small ponds heat up quickly. A diffuser aerator running continuously in summer and at night helps protect against hot periods, when the lack of oxygen is felt most strongly in the early morning.

Quality food, in small quantities. Well-digested food produces less waste. Feed only what the fish eat in a few minutes, and adjust the ration to the water temperature.

The order of introduction

A new pond is not ready on the day it is filled. The bacteria that transform ammonia and nitrites into nitrates take several weeks to become established, a little faster if the water is warm and the filter is seeded. A common scenario: a pond filled in May, stocked with fifteen fish the following weekend, followed by cloudy water and lethargic fish two weeks later. It is better to introduce a small first group after the filter has been running for three or four weeks, then add more gradually. Regular water testing during this first month avoids working blindly.

Read also

To refine the calculation: How many fish should you keep in a pond?.

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Frequently asked questions

— Can you keep koi in a 4500-liter pond?

Temporarily, a few juveniles can. As adults, koi need a significantly larger volume and more powerful filtration. If you dream of keeping koi, plan to expand or use another pond.

— Is a filter mandatory for 4.5 m³?

Not necessarily for a few fish in a heavily planted, lightly fed pond. As soon as you exceed five or six adult fish, biological filtration becomes very useful for keeping the water stable.

— How deep should the pond be for fish to overwinter?

Aim for about 80 cm in one part of the pond. This area keeps the water more stable in winter and provides a cool refuge in summer.

— What should you do when goldfish multiply too much?

Catch the juveniles with a hand net or fish trap and offer them to other pond owners or an aquatic garden center. Never release them into a waterway or natural pond.

In summary

• 4500 liters is suitable for goldfish, shubunkins, comets, and minnows.
• Koi and sturgeon grow too large for this volume.
• Rule of thumb: 10 to 15 adult goldfish with good filtration.
• Plan for reproduction, introduce fish gradually, and provide aeration for the summer.

Last updated: September 29, 2026.