Aquaponics Fish: The Complete Beginner’s Guide

Fish are the heart of every aquaponics system. They produce the nutrients that feed your plants — and if you choose the right species and care for them well, they’ll reward you with healthy harvests and, eventually, a plate of delicious home-grown fish.

This complete guide covers everything a beginner needs to know about aquaponics fish: which species to choose, how to set up and stock your tank, how to feed and care for your fish, and how to harvest them when the time comes.

The Role of Fish in an Aquaponics System

In aquaponics, fish and plants exist in a mutually beneficial relationship. Here’s the cycle:

1. You feed the fish.
2. The fish produce waste (rich in ammonia).
3. Beneficial bacteria convert the ammonia into nitrates — the ideal fertiliser for plants.
4. The plants absorb the nitrates, cleaning the water in the process.
5. The clean water returns to the fish tank, ready to start again.

Without fish, there are no nutrients. Without plants, the water becomes toxic to the fish. Together, they create a self-sustaining ecosystem that is more productive — and far more sustainable — than growing either fish or vegetables alone.

Best Fish Species for Aquaponics

Not every fish is suited to aquaponics. The ideal aquaponics fish is hardy, grows well in the relevant temperature range, is easy to source, and tastes great on the plate. Here are the most popular choices:

Tilapia

Tilapia is the world’s most popular aquaponics fish, and for good reason. It is extremely hardy, tolerates a wide range of water conditions, grows fast, and tastes mild and pleasant. If you live in a warm climate, tilapia is hard to beat. Note: tilapia is illegal to keep in some Australian states — check your local regulations before sourcing.

Barramundi

Barramundi is an excellent choice for Australian growers in warm climates. It is a prized food fish with firm, delicious white flesh. Barramundi grow quickly in warm water (above 24°C) but are sensitive to cold. They are a great option if you can maintain water temperatures in the right range.

Silver Perch

Silver perch is well suited to cooler Australian climates. It is a native species, tolerant of lower temperatures and varying water quality, and produces good-quality fillets. Silver perch grows more slowly than tilapia but is a reliable, low-maintenance choice.

Trout

If you live in a cooler climate, trout is one of the best aquaponics fish available. It thrives in cold, well-oxygenated water (12–18°C), grows to a good size, and is one of the finest eating fish you can raise at home. Trout are more sensitive to water quality than tilapia or silver perch, so they suit growers who are comfortable with regular water testing.

Goldfish and Koi

Goldfish and koi are excellent choices for ornamental aquaponics setups — systems where plant production is the priority and you don’t intend to eat the fish. They are extremely hardy, widely available, and produce plenty of waste to fertilise your plants. A great starting point for beginners who are new to keeping fish.

Setting Up Your Fish Tank

Your fish tank is the foundation of your system. Getting the setup right from the start will save you significant trouble down the track.

Tank Size

As a general rule, allow a minimum of 40–50 litres of water per kilogram of fish at maximum stocking density. For a beginner system, a 500–1,000 litre tank is a practical starting point. Bigger tanks are more stable: they buffer temperature swings and water quality fluctuations more effectively than small tanks.

Tank Material

The best tank materials for aquaponics are food-grade polyethylene (plastic) or fibreglass. Both are non-toxic, durable, and easy to clean.

• Avoid metal tanks — they can leach toxins into the water, which is dangerous for both fish and the humans eating the plants.
• Avoid untreated concrete — concrete raises water pH significantly, making it harder to maintain the ideal range of 6.8–7.2. If you must use a concrete tank, seal it with a food-safe rubber pond liner or sealant.
• Recycled IBC containers are popular and cost-effective — but always verify what the container previously held. Avoid any IBC that stored chemicals, mineral oils, or resins.

Positioning Your Tank

Place your tank in a spot with some shade to prevent excessive algae growth and to keep water temperatures stable. Avoid placing it directly under trees where falling leaves can decompose in the water.

How Many Fish Should You Keep?

Overstocking is one of the most common beginner mistakes. Too many fish produces more ammonia than your bacteria and plants can process, leading to dangerous water quality conditions.

A safe starting stocking rate is 1 kg of fish per 40–50 litres of water. Start lightly stocked and build up gradually as your system matures and your bacteria colony grows stronger. A good rule of thumb: if in doubt, add fewer fish, not more.

Fish Care and Water Quality

Healthy fish require clean, well-oxygenated water within the right temperature range. Test your water regularly — at least once a week when your system is new, and once every two weeks once it’s established.

The key water parameters to monitor are:

• pH: 6.8–7.2 (the sweet spot for fish, bacteria, and plants)
• Ammonia: Below 0.5 mg/L — high ammonia is toxic to fish
• Nitrites: Below 0.5 mg/L — nitrites are also toxic; your bacteria should keep these near zero
• Nitrates: Below 100 mg/L — your plants will consume these happily
• Dissolved oxygen: Above 5 mg/L — use an air stone or surface agitation to maintain adequate oxygenation
• Temperature: Varies by species (see above) — stable temperature is more important than hitting a precise number

What to Feed Your Aquaponics Fish

Remember: your fish food is your fertiliser. Whatever you feed your fish becomes the nutrients that feed your plants. High-quality fish pellets produce a full spectrum of nutrients; cheap, low-quality food produces a poorer nutrient profile.

Feed your fish once or twice a day, and only as much as they will eat in 5 minutes. Remove any uneaten food to prevent it from decomposing and spiking your ammonia levels. As your fish grow, increase the quantity of food proportionally.

For a more sustainable system, you can supplement commercial pellets with duckweed, black soldier fly larvae, or homemade fish food — all of which can be produced cheaply at home.

When to Add Fish to Your System

Never add fish to a brand-new aquaponics system. A new system has no established bacteria colony, which means ammonia from your fish will accumulate rapidly and can kill them within days.

Before adding fish, you need to “cycle” your system — the process of building up the beneficial bacteria population that converts ammonia to nitrates. This typically takes 4–6 weeks. You can speed the process by using a bacterial starter culture, adding a small amount of fish food to produce ammonia, and testing your water daily until ammonia and nitrite levels drop to near zero.

Once your system is cycled, start with a small number of fish — around 30–40% of your eventual target stocking density — and build up gradually over the following weeks.

Where to Buy Aquaponics Fish

Source your fish from a reputable local supplier, aquaculture farm, or dedicated aquaponics supplier. Avoid buying fish from pet shops for food production; these fish may be treated with chemicals that are not food-safe.

Before purchasing, ask the supplier about the health history of the stock, what they have been fed, and whether they have been treated with any medications. When transporting fish, minimise stress by keeping them cool, well-oxygenated, and in the dark.

How to Harvest Aquaponics Fish

Harvesting is one of the most rewarding moments in aquaponics. Most fish reach harvest size (around 400–600g for a plate-sized fish) between 6 and 18 months, depending on the species, water temperature, and feeding rate.

To harvest, use a net to catch the fish directly from the tank. The most humane method of slaughter is a sharp blow to the head (called “iki jime” in Japanese practice), followed by immediate bleeding. This method also produces the best-tasting fish.

You don’t need to harvest all your fish at once. Remove the largest fish first, which creates space and reduces competition for the smaller fish still growing.

Alternatives to Fish in Aquaponics

Fish are the classic choice, but they’re not the only option. Other aquatic animals that work well in aquaponics systems include:

• Freshwater crayfish (yabbies) — produce good nutrient output and are excellent eating
• Freshwater prawns — suited to warm-climate systems
• Ducks — their pond waste can fertilise a connected aquaponics system (a technique sometimes called “aquaducks”)

These alternatives can be used alone or alongside fish, depending on your setup and local regulations.

Start Growing Your Own Fish and Vegetables

Aquaponics fish are far easier to raise than most people expect. With the right species, a properly sized tank, good water management, and quality food, your fish will thrive — and so will your plants.

The hardest part is getting started. Once your system is up and running, it quickly becomes second nature.

→ Download our free aquaponics starter guide and learn how to build your first system step by step.