Which fish to choose for aquaponics ?

The goldfish is arguably the most widely used species in aquaponics. It can live in a small tank and tolerate significant temperature fluctuations. Hardy and very forgiving, goldfish adapt well to temperate or cold water.

Koi carp, being larger, are ideal in an outdoor pond and add a decorative touch with their vivid colours. These fish are not intended for the table, but they contribute effectively to the nitrogen cycle by producing waste that nourishes the plants. Simply take care not to overstock the pond to keep the water clean — good filtration and regular water changes help maintain the balance.

One final advantage of ornamental fish is worth highlighting: they are available everywhere, inexpensive, and easy to replace. For a first system, that is a genuine comfort, as the early stages almost always involve trial and error with filtration and the nitrogen cycle. It makes sense to learn the ropes with forgiving fish before investing in more demanding species.

For very small systems or aquariums dedicated to aquaponics, you might look at small tropical fish. Guppies, for example, are livebearing fish that are highly resilient and straightforward to keep. They breed quickly, which can be fascinating to watch. These species can adapt to cooler water and can be kept outdoors from April to October in most regions, though you will need to plan a winter solution in advance.

Although their nutritional impact on the plants is more modest, they make a fun and educational choice for beginners. Other aquarium fish such as bettas, gouramis, ancistrus, and certain cichlids can also work well in this context.

One word of caution about smaller formats: a small volume goes out of balance far more quickly than a large one. Just a few litres is enough for ammonia to spike within a day if feeding is too generous, and temperature swings are also far more abrupt. It is therefore well worth understocking significantly at the outset and testing the water frequently, only increasing the population once the cycle is properly established.

The April to October window applies to a temperate climate. It extends by several weeks around the Mediterranean and shortens accordingly in Scandinavia and central Europe: trust the water temperature you measure at your own location rather than the calendar months.

The rainbow trout is prized for its delicate flesh and rapid growth. It requires cool, well-oxygenated water, typically between 10 and 18 °C, making it ideal for systems with precise temperature and aeration control — though it can be challenging to maintain in summer.

The brook char, also known as brook trout, is in fact neither a trout nor a salmon despite its two common names: it is a char, a close relative within the salmonid family. It is somewhat hardier and more tolerant of water quality variation than rainbow trout. It generally performs best between 13 and 16 °C and begins to struggle above 20 °C. Its growth is slightly slower, but it is more forgiving of management errors, making it an excellent choice for a first cold-water fish-keeping project.

The grayling is the most demanding of the three. This fast-flowing river fish requires cold, highly oxygenated, moving water. It is suited only to very well-equipped systems and is more relevant to river restocking than to domestic aquaponics, but it is part of the fabric of our waterways and well worth knowing about.

Finally, the common carp, like koi carp, stands out for its robustness. Omnivorous and highly adaptable, it fits well into aquaponic systems and tolerates both warm and cold water. It is ideal for straightforward setups, particularly outdoors or where food self-sufficiency is the goal.

The right choice will depend on your local climate, your objectives, and the technical constraints of your aquaponic system. A useful benchmark is to measure your water temperature at the hottest point of August before ordering anything: it is that figure, not the annual average, that determines what you can realistically keep.

Common perch (Perca fluviatilis) and pike-perch (Sander lucioperca) belong to the same family, the Percidae. They behave in much the same way, and are arguably the most promising freshwater fish for European aquaponics: prized flesh, species naturally present in our waters, and closed-system rearing that is now well documented by research.

In terms of management, perch requires warmer water than trout: growth is generally optimal between 22 and 24 °C, making it far easier to keep through summer than a salmonid. It does, however, need well-oxygenated water, around 6 mg/L of dissolved oxygen, roughly 70% saturation. Pike-perch follows the same thermal logic, with a shyer temperament and a marked preference for shaded areas of the pond.

Both are predatory fish, which calls for a suitable diet based on high-protein pellets. Farmed specimens already weaned onto pellets are considerably easier to manage than fish taken from the wild, which often refuse inert feed.

One point worth knowing before you start: in Percidae, the fastest-growing individuals may attack smaller ones. This is not insurmountable, but it is worth sorting fish by size periodically and separating batches that have become too uneven, especially with young specimens.

The sturgeon is a bottom-feeding fish: it rummages through the grow medium and rarely comes to the surface to feed. It is best suited to slow-sinking pellets formulated for its species, a clear pond floor, and a generous water volume. It generally overwinters well, but struggles when water temperatures remain above 25 °C for extended periods, as warmer water holds less dissolved oxygen. In aquaponics, it is best kept in large, deep outdoor ponds, where it coexists happily with koi carp.

The wels catfish sits at the opposite end of the spectrum. Fry are typically pre-grown at 25 to 28 °C, with the grow-out phase conducted at 20 to 22 °C. Hardy and tolerant of variable water quality, it accepts high stocking densities and can be kept with others of its kind in the same tank, provided all individuals are of a similar size. It is, however, a fairly demanding species to manage: it consumes large quantities of feed, takes time to reach a marketable size, and requires close monitoring of dissolved oxygen levels — all the more so because it prefers warm water, which retains less oxygen. A well-secured tank lid is also essential.

Finally, do not overlook coarse fish. Roach and tench are inexpensive, highly resilient, and perfectly suited to our local temperatures, meaning they can be kept outdoors year-round without any heating. They produce nitrate steadily and cohabit very well with ornamental fish such as goldfish or koi carp, giving you the flexibility to vary the pond's population.

Tench, which forage along the bottom, and roach, which are more active in open water, occupy two distinct levels of the pond and complement each other well. For a first outdoor setup, this is one of the most straightforward combinations to get right.

Tilapia is the emblematic species of tropical aquaponics. Easy to raise, undemanding in terms of oxygen and highly resilient to fluctuations in water quality, it thrives between 22 and 30 °C. Its rapid growth and ease of reproduction make it a strategic choice for large-scale production. It is also an omnivore, which greatly simplifies feeding, as it readily accepts predominantly plant-based pellets that are less costly than carnivore formulations.

The African catfish, or clarias, tolerates difficult conditions, including oxygen-poor water, thanks to an accessory respiratory organ that allows it to gulp air at the surface. It is carnivorous and grows quickly, making it an interesting species for well-balanced systems. It does, however, require a securely covered tank, as it escapes readily.

The Nile perch, a much larger fish, is suited to big ponds. It offers a highly prized flesh, but demands careful water management and a specific diet, which in practice limits it to professional installations.

A word of caution, however. At our latitudes, these species require heating the water for a significant part of the year, with the corresponding energy costs, and they are subject to specific regulations as exotic species that must be kept in a closed circuit with no discharge whatsoever into natural waterways. They are therefore best suited to indoor settings, in small, well-insulated volumes, or in a heated greenhouse. For an outdoor project, native river species remain considerably easier to manage and far less costly to run.

Not all species cohabit well together: some are aggressive, others are too sensitive or become stressed in the presence of different species. Their requirements for temperature, oxygen, or food may also be incompatible. For example, keeping goldfish with tilapia, or trout with tropical fish, carries real risks.

If you wish to introduce several species, choose fish of a similar size with peaceful temperaments and compatible living conditions — such as koi carp and goldfish.

The safest combination is one that pairs a mid-water fish with a bottom-dwelling fish: for instance, a roach and a tench, or a koi carp and a sturgeon in a large pond. Each occupies a different level, and competition for food remains limited.

That said, it is often simpler to stick to a single species per system in order to ensure stability and ease of maintenance — particularly during the first year.

Choose a quality fish food suited to the species you are raising. For example, high-protein pellets for carnivores such as trout, perch or catfish, and plant-based pellets for omnivores such as tilapia or carp.

As a general rule, it is recommended to feed small amounts once or twice a day, which the fish can consume within a few minutes. Remove any leftovers with a net to avoid polluting the water.

Adjust rations according to the season too. In cold water, fish metabolism slows considerably, and a summer ration given in November ends up on the bottom of the tank, where it pollutes the water without benefiting anyone. Below 10 °C, most temperate species feed very little, and this is perfectly normal.

A good nutritional balance promotes fish growth and limits waste production, which eases the load on the filtration system and keeps the water cleaner.

Fish stocking density is a fundamental parameter in aquaponics. Too many fish, and you face waste overload, water pollution, stress, and disease. Too few, and the plants are short of nutrients.

In a domestic aquaponic system, the balance is often found at around 15 to 20 kg of fish per cubic metre of water, though this depends on the species, the filtration, and the quantity of plants being grown.

Always start with a moderate density. Monitor how nitrate levels and fish condition evolve. If the plants are lacking nutrients, you can gradually increase the population. When in doubt, fewer healthy fish is always preferable to too many animals under stress.

Bear in mind, too, that density must always be considered alongside growing area. A well-stocked pond connected to too few plants will accumulate nitrates, whereas a large growing bed fed by too few fish will produce pale, chlorotic vegetation. It is the ratio between the two, far more than the number of fish taken in isolation, that determines the stability of a system.