Fish for Aquaponics: 7 Easy-to-Raise Species and How to Choose Based on Your Climate

In aquaponics, the fish is not merely decoration: it is the engine of the system. Its waste, converted into nitrates by bacteria, feeds the lettuces, tomatoes or herbs, which in turn purify the water. Choosing the right species therefore means choosing the reliability of the entire setup. For beginners, the best fish are those that can withstand temperature fluctuations, somewhat nutrient-rich water and minor feeding mistakes: goldfish, carp or koi, and tench first, followed by tilapia, trout, perch and ide depending on your climate and goals. Here is how to decide, species by species, without getting off to the wrong start.

What is expected of an aquaponics fish

An ornamental aquarium fish and an aquaponics fish do not live the same kind of life. The latter lives in water that circulates in a loop between a rearing tank, a filter and grow beds. It must tolerate water with higher nitrate levels than a well-maintained aquarium, temperatures that often follow those of the room or greenhouse, and a pH that slowly drifts toward acidity as nitrification progresses. It must also eat readily, since its food, once digested, fertilizes the plants.

In practical terms, four criteria make a good aquaponics fish for a private individual:

• a wide temperature tolerance, so you do not depend on costly heating;
• resistance to changes in water quality, particularly while the biological filter is starting up;
• a simple diet, with commercial pellets and no live prey;
• a clear legal status: the species must be allowed to be kept and transported without restrictions.

The choice then depends on your goal. Some people only want vegetables and see the fish as a living "fertilizer maker." Others also want to harvest fish to eat. Both approaches are legitimate, but they do not lead to the same species. If the very principle of the system is still unclear to you, our article What is aquaponics and how does it work? lays out the basics.

The 7 easy fish, at a glance

The table below summarizes the needs of each species. The temperatures are approximate ranges, not absolute thresholds: the same species may tolerate slightly more or slightly less depending on its acclimatization, age, and water quality.

1. The goldfish: the most forgiving

The goldfish (Carassius auratus) is often underestimated by aquaponic growers, wrongly so. It is probably the most forgiving fish for running a first system. It tolerates cold water in winter and lukewarm water in summer, and it continues to eat in conditions that would stress most other species. Its modest price also lets you "pay for" beginner mistakes without serious consequences.

Its limitation is obvious: you don't eat it. It is therefore ideal if your main goal is the vegetable garden. It produces a lot of waste for its size, which, in this case, is more of a quality. Just be careful not to overload the tank: a few dozen adult goldfish are more than enough for a small garden system.

2. The common carp or koi: hardy and long-lived

Common carp (Cyprinus carpio) and its ornamental form, koi, are safe choices. They tolerate a very wide range of temperatures, spend the winter outdoors in our latitudes, and feed on standard floating pellets. They grow steadily and live a long time, making them long-term “partners” for a system installed in an unheated greenhouse or outdoors.

Two practical points. First, carp forage: if your tank contains substrate or plants, they will stir it up. In aquaponics, this is not a problem as long as the prefilter retains the particles. Second, common carp can be eaten, but the taste depends greatly on the water; in a clean, well-filtered system, the flesh is significantly better than that of a muddy pond carp. Koi, on the other hand, remains an ornamental fish: it is chosen for its beauty and longevity.

3. Tench: the overlooked hardy fish

The tench (Tinca tinca) is a fish native to our calm waters, renowned for tolerating difficult conditions, including oxygen-poor water and bottoms rich in organic matter. It grows more slowly than carp, remains discreet, and often lives near the bottom. For a rustic outdoor system, it is an interesting companion, possibly alongside goldfish or carp.

Its quiet nature has a downside: you see it less often, and it is more difficult to tell whether it is eating well. Use feed that sinks partially, or distribute part of the ration at the end of the day, when it is more active.

4. Tilapia: the classic choice for heated systems

Nile tilapia (Oreochromis niloticus) is the fish most widely used in aquaponics worldwide. It grows quickly, tolerates average water quality, eats anything, and reproduces easily. Its mild, white flesh appeals to most palates.

But it is a tropical fish. Below around twenty degrees, it eats less and barely grows; water that drops toward twelve degrees quickly becomes dangerous for it. In Belgium and northern France, it is therefore only feasible indoors, in an insulated space with water heating, or outdoors during the warmest months only, with a plan for autumn. If this is your project, our tilapia guide provides details on heating, stocking density, and breeding.

5. Rainbow trout: for cool waters

The rainbow trout (Oncorhynchus mykiss) is the opposite of tilapia: it likes cool water and suffers when the water warms up. It is a good option for systems installed in a cellar, basement or outdoors during the cold season. Its flesh is excellent.

However, it is more demanding than it may seem. It needs plenty of dissolved oxygen, clean water and flow. An air pump failure lasting a few hours, a spell of summer heat or a rise in nitrite levels can be fatal. It is not a fish for a first attempt, but an excellent choice for an aquaponist who has already run a system for one season and has mastered aeration. The role of oxygen in these systems is explained in our article How oxygen works in aquaponics.

6. The common perch: native and delicious

The perch (Perca fluviatilis) is a fish from our rivers and lakes, with delicate and highly prized flesh. It adapts well to farming, provided it is accustomed to pellets from a young age: buy perch already weaned onto dry feed, otherwise they may turn their noses up at the food.

It is a predator. Do not mix it with small fish that it might consider prey, and avoid mixing perch of very different sizes. It prefers clear, well-oxygenated water with shaded areas. It is best reserved for a second project, once you have mastered running the system.

7. The ide: the lively fish for an outdoor pond

The ide (Leuciscus idus), in its golden or silver form, is a very active fish that swims in schools near the surface. It tolerates the cold well, enjoys flowing water and is highly visible, making it a pleasant choice for an outdoor system or a decorative aquaponic pond. It needs plenty of space and well-oxygenated water; in a small tank without flow, it quickly feels cramped.

It is not a commonly consumed fish here, but it brings life and plays a full part in the nitrogen cycle.

Species to avoid at all costs

Some species are easy, even too easy. Several hardy fish appear on the list of invasive alien species of concern to the European Union, drawn up under Regulation (EU) 1143/2014. Their possession, transport, breeding and trade are prohibited, even in closed systems. They include:

• the pumpkinseed (Lepomis gibbosus), once often recommended for ponds;
• the black bullhead (Ameiurus melas), hardy and prolific;
• topmouth gudgeon (Pseudorasbora parva), which sometimes slips in among batches of juvenile fish;
• mosquitofish, as well as several American crayfish species.

The list changes: check it before buying an uncommon species. And whatever the species, never release aquaponics fish into the wild, a pond or a watercourse.

Choosing according to your installation: the method

Rather than starting with the species, start with your installation. Three questions are enough to guide your choice.

What will the water temperature be in winter?

This is the question that narrows the choice down fastest. If the water drops below around ten degrees, forget tilapia. If it regularly rises above twenty degrees in summer, trout will suffer. Measure the temperature of your room or greenhouse over a full season before deciding: an unheated greenhouse can exceed thirty degrees in summer and freeze in winter.

Do you want to eat the fish?

If so, the candidates are tilapia in a heated indoor setup, trout in cool water, perch and, potentially, carp. If not, goldfish, koi, tench or ide make life easier: you do not have to manage slaughter or stock replacement.

How much safety margin do you have for oxygen?

A home system runs on two electrical devices: the circulation pump and the air pump. The more oxygen-demanding the species (trout, perch, ide), the more you need to provide reliable aeration, ideally with a backup. Hardy fish (carp, tench, goldfish) give you more time to react in the event of a failure.

Starting without losing fish

Most losses in aquaponics occur during the first few weeks, when the biological filter has not yet become established. Ammonia and nitrites, produced by the fish and their food, have not yet been converted into nitrates. A common scenario: a filled system, fish introduced the next day, generous feeding with pellets, and water that becomes toxic after one or two weeks.

To avoid it:

1. Let the system run for several weeks before introducing most of the fish, or start with very few hardy fish.
2. Test the water two to three times a week during the first month: ammonia, nitrites, pH, nitrates.
3. Feed sparingly, once or twice a day, only what is consumed within a few minutes.
4. Increase the stock in stages, checking each time that nitrites remain close to zero.

For stocking density, be cautious: a few kilograms of fish per cubic meter for a first system is more than enough to feed a few lettuce beds. High-density systems exist, but they require appropriately sized filtration and aeration, along with nearly daily monitoring.

Matching fish and plants

The final adjustment is the balance between what the fish eat and what the plants consume. A system with many fish and few plants accumulates nitrates; the opposite results in pale, nutrient-deficient plants. We usually base this on the amount of feed distributed per day in relation to the cultivated area: a few dozen grams of feed per day per square meter of grow bed is a common guideline for leafy vegetables, and more for fruiting vegetables such as tomatoes.

pH is the other point of balance. Fish and bacteria prefer neutral to slightly alkaline water, while plants absorb certain elements more effectively in slightly acidic conditions. A compromise around neutral is generally the goal. All the species mentioned here tolerate this compromise. To choose the crops that come next, our selection of the 10 best plants for aquaponics is a good starting point.

Finally, monitor behavior. A healthy aquaponics fish eats eagerly, swims normally, and does not remain stuck at the surface. Fish that “gulp” air at the surface, stay near the water inlet, or refuse food almost always indicate an oxygen or water-quality problem, well before a disease develops.

Also worth reading

Are you aiming for indoor fish production? Read our detailed guide: Raising tilapia in aquaponics.

READ THE ARTICLE

Frequently asked questions

— What is the easiest fish for beginners in aquaponics?

Goldfish and carp (common carp or koi) are the most tolerant: they can handle both cold and lukewarm water, imperfect water quality, and a few feeding mistakes. They are not intended for consumption, but they keep the system running reliably.

— Can tilapia be raised in Belgium without heating?

Not all year round. Tilapia are tropical fish that slow down considerably below around 20°C and do not survive long in cold water. Outdoors, they are suitable only during the warmer season; to raise them year-round, you need a heated, insulated indoor setup.

— How many fish should be added to an aquaponic system?

Start small: a few kilograms of fish per cubic metre of water are more than enough for a first system. Then increase gradually while monitoring ammonia and nitrites and maintaining a consistent ratio between the feed provided and the growing area.

— Which fish are prohibited in aquaponics?

All species listed on the European list of invasive alien species (EU Regulation 1143/2014), including pumpkinseed, black bullhead, topmouth gudgeon, and mosquito fish. Keeping, transporting, and trading them are prohibited.

In summary

• Fish are the driving force behind aquaponics: choose them above all for their hardiness.
• Without heating, in our latitudes: goldfish, carp or koi, tench.
• With heated indoor water: tilapia; with cool, highly oxygenated water: trout.
• Perch and ide are suitable for well-managed outdoor systems with good aeration.
• Never use invasive alien species prohibited under EU Regulation 1143/2014.
• Start with a few fish, test the water, and increase gradually.

Last updated: October 3, 2026.