What Is Aquaponics? How It Works, Step by Step

Aquaponics is a way of growing fish and vegetables together in one recirculating water system. The fish feed the plants and the plants clean the water for the fish. Readers ask me how does aquaponics work more than any other question, so this page answers it properly: what happens to the water at every step, which parts a system needs, and the numbers that keep fish, bacteria and plants in balance.

The explanations stay practical, and every figure points to the extension publication behind it, so you can check them yourself before you build anything.

What Is Aquaponics?

Aquaponics is the combined culture of fish and plants in a recirculating system. The University of Florida's A Practical Guide for Aquaponics as an Alternative Enterprise (UF/IFAS HS1252, 2014) describes it as an intensive, sustainable production system that connects hydroponic and aquaculture systems to produce more than one crop with reduced water and fertilizer inputs. The name says the same thing: aqua from aquaculture and ponics from hydroponics.

In a fish-only recirculating system, waste builds up and part of the water has to be dumped and replaced every day. In a hydroponic garden, nutrients come from bags and bottles of fertilizer. Aquaponics links the two so that the waste of one becomes the input of the other. SRAC Publication No. 454 (Rakocy, Masser and Losordo, 2006 revision), the Southern Regional Aquaculture Center fact sheet written by the team behind the University of the Virgin Islands (UVI) system, notes that in closed systems with very little daily water exchange (less than 2 percent), dissolved nutrients build up to concentrations similar to hydroponic nutrient solutions. Those nutrients are what the plants live on.

Three Living Parts in One System

Three groups of organisms do the work, and the UF guide lists them the same way: the fish, the plants and the nitrifying bacteria. The fish and the plants are the two crops. The bacteria are invisible but just as important, because they turn toxic fish waste into a form plants can use. Many home growers also add red worms to media beds; SRAC 454 describes UVI's gravel beds being inoculated with red worms, which improve aeration and break down organic matter.

How Does Aquaponics Work?

Aquaponics works as a loop. You feed the fish, the fish release ammonia, bacteria convert it to nitrate, plants take up the nitrate, and the cleaned water returns to the fish tank. A single pump keeps the water moving around that circle. Here is the loop, then each step in more detail.

Step 1: Fish Eat and Release Ammonia

Everything starts with fish feed. Fish digest the protein in their food and excrete ammonia, mostly through their gills, plus solid waste. The UF guide gives a handy rule of thumb: about 10 percent of the protein in fish feed ends up as ammonia nitrogen in the water. In their worked example, 1 kg of feed with 30 percent protein produces 27.6 g of nitrogen in a day. That is why feeding rate, not fish count, is the number system designers size everything else around.

Step 2: Bacteria Turn Ammonia Into Nitrate

Ammonia is toxic to fish, so it cannot be left in the water. Nitrifying bacteria living on every wet surface (grow media, tank walls, plant roots, biofilter media) oxidize it in two stages: first to nitrite, which is also toxic, and then to nitrate, which SRAC 454 calls relatively non-toxic. This process is called nitrification, or biofiltration.

The bacteria take time to establish. The UF guide puts it at a 3 to 4 week process that should be finished before you add many fish, which is why new systems are "cycled" first. Nitrification also likes oxygen and a pH on the alkaline side: SRAC 454 gives an optimum range of 7.0 to 9.0 for the bacteria.

Step 3: Plants Take Up the Nitrate

Nitrate is one of the main nutrients plants need, so the grow bed acts as the filter that finishes the job. Plant roots absorb nitrate and other dissolved minerals, and the water that leaves the grow bed is cleaner than the water that arrived. In a well-designed system, SRAC 454 explains, the hydroponic part can do the biofiltration on its own, which removes the cost of a separate biofilter.

Fish feed does not supply everything plants need in the right amounts. The UVI system, SRAC 454 notes, needs supplements of potassium, calcium and iron, and the UF guide says the same for calcium, potassium and iron. When iron runs short, it shows as yellowing between the veins of the newest leaves.

Step 4: Clean Water Returns to the Fish

The filtered water drains back to the fish tank, usually by gravity, and the cycle starts again. SRAC 454 recommends just one pump to circulate water in an aquaponic system, with the sump as the only tank where the water level drops. Water still leaves the loop through evaporation, plant transpiration and solids removal, so you top up with fresh water. If your top-ups suddenly jump, look for a leak.

What Are the Parts of an Aquaponics System?

Whatever the size, an aquaponics system has the same building blocks. A countertop kit squeezes them into one unit; a backyard system spreads them across separate tanks and beds.

Fish Tank

The tank should be sturdy, watertight and made from a material that will not leach anything into the water, such as food grade plastic or a purpose-made aquarium. Round or oval tanks avoid dead corners where solids settle. Keep the tank mostly covered: the UF guide advises it to keep out sunlight and reduce algae. For stocking, the same guide calls one pound of fish per cubic foot of tank space (about 7.5 gallons) a reasonable density for small farms, with higher densities only when you can monitor around the clock and have backup power. On my own backyard system I started well below that, with one fish for every 10 gallons, and added fish as the system proved it could keep up.

Grow Beds and Growing Media

The grow bed holds the plants. In a media bed it is filled with an inert material that supports roots and houses bacteria. SRAC 454 recommends pea gravel of 1/8 to 1/4 inch for flood and drain gravel systems. Perlite, gravel and expanded clay are the common choices: perlite is light enough to raise beds off the ground easily, gravel is cheap but heavy, and clay pebbles sit in the middle on weight and cost. If you are choosing a medium, our guide to clay pebbles for aquaponics compares the options.

Solids Removal and the Biofilter

Fish produce solid waste as well as ammonia. If too much of it reaches the plants, it rots, uses up oxygen and can even release ammonia instead of removing it, as SRAC 454 warns for overloaded gravel beds. Small media-bed systems often let the gravel handle solids, while systems with more fish add a settling tank or filter before the plants. The biofilter is any surface where nitrifying bacteria can colonize; in a media bed, the bed is the biofilter.

Sump, Pump and Aeration

The sump sits at the lowest point and holds the pump that sends water back up to the fish tank or beds. Fish and roots both need dissolved oxygen. The UF guide lists low oxygen (under 3 ppm) and prolonged high ammonia (over 2 ppm) as the most common causes of fish deaths, so an air pump with air stones is cheap insurance, especially in warm water, which holds less oxygen.

Three Ways to Grow the Plants

The fish side of aquaponics is much the same everywhere. The plant side comes in three main designs, and the UF guide names the same three as the most common: media beds, floating rafts and nutrient film technique (NFT).

For a first DIY project I still recommend a media bed. It is the most forgiving design, the bed does the filtering, and the UF guide notes that media systems such as bench beds have the broadest crop choices. Rafts make sense once you want to grow a lot of lettuce, and NFT channels are best left until you have a separate filter.

How Big Should the Grow Bed Be?

The balance between fish and plants is set by how much feed goes in each day compared with how much plant area takes nutrients out. SRAC 454 gives two starting points: for raft systems, 60 to 100 g of fish feed per square meter of plant growing area per day, and for flood and drain gravel systems, a volume ratio of 1 cubic foot of fish tank to 2 cubic feet of pea gravel. Gravel and NFT systems should run at roughly a quarter of the raft feeding ratio, because they hold far less water. Too much feed for the plant area and nutrients build up; too little and plants show deficiencies.

The Numbers That Keep the System in Balance

Fish, bacteria and plants each prefer slightly different water, so an aquaponics system runs on compromises. These are the targets the extension guides give, and the ones worth testing for every week.

The UF guide recommends testing ammonia, pH, oxygen and nitrite at least once a week, and more often during startup. A liquid test kit covers the main ones.

Starting a New System

Because the bacteria take weeks to establish, new systems are started before the fish arrive, or with only a few fish. The UF guide sets out one startup schedule: fill and adjust pH to 7.0 to 7.5, run the pumps, add a quarter-strength hydroponic nutrient mix and household ammonia to 3 to 5 ppm, add a nitrifying bacteria culture, plant from day five, and add fish in weeks three to four once ammonia and nitrite read near zero. After startup, the fish supply all the ammonia the bacteria need.

If you use municipal water, remember it contains chlorine or chloramine, which the UF guide calls very toxic to fish. It recommends aerating city water in a separate tank for a few days and testing for chlorine before fish touch it.

What Can You Grow With Aquaponics?

Leafy greens and herbs are the easiest crops because they use modest amounts of nutrients and grow fast. SRAC 454 calls lettuce a good aquaponic crop because it spends only 3 to 4 weeks in the system and so has relatively few pest problems. Basil was one of the strongest crops in UVI's trials. Fruiting plants such as tomatoes, peppers and cucumbers also grow well, especially in media beds, but they need more nutrients, more time and more pest watching. Our list of the best plants for aquaponics goes crop by crop.

On the fish side, SRAC 454 says tilapia are the species most commonly raised, with channel catfish, bass, crappie, trout, carp, koi and goldfish among the others used. Your climate and water temperature decide much of the choice: the UF guide notes tilapia stop feeding at 60°F and begin to die at 50°F and below. See our guide to the best fish for aquaponics before you buy any.

Benefits and Downsides of Aquaponics

Benefits

• Two crops from one input. Fish feed produces both fish and vegetables.
• Very little water is discharged. The UF guide explains that a fish-only recirculating system typically discharges 5 to 10 percent of its water daily, while an aquaponic system with enough plants can run at near zero discharge.
• No soil. No digging, weeding or tilling, and systems can sit on patios, in garages or in greenhouses.
• No pesticide residues. Because pesticides cannot be used near fish, the crops are grown without them.

Downsides

• Setup cost and complexity. SRAC 454 is blunt that commercial systems have a high capital cost and must run near full capacity to make a profit. Home systems cost more than a soil bed of the same size.
• A slow start. Expect 3 to 4 weeks before the bacteria are ready for a full load of fish.
• Compromise water. pH near 7.0 is not ideal for every plant, and potassium, calcium and iron often need topping up.
• No chemical pest control. SRAC 454 says pesticides should not be used on aquaponic plant crops because they threaten the fish, so you rely on screens, traps and beneficial insects.
• Living animals. Fish need feeding every day and a power cut that stops the air pump can kill them.

Aquaponics vs Hydroponics vs Soil

Hydroponics grows plants in water too, but the nutrients come from mineral fertilizer instead of fish. That makes hydroponics simpler to control and quicker to start, while aquaponics adds fish as a second crop and replaces most of the fertilizer bill with fish feed. Compared with soil gardening, both use far less water and no weeding, but need power and equipment. We cover the trade-offs in detail in aquaponics vs hydroponics.

How to Get Started

There are two sensible first steps. If you want to see the cycle working before you commit, a small tank-top unit from our aquaponics kits guide shows the loop on a counter. If you want food, build a media-bed system: our step-by-step guide on how to build an aquaponics system covers tank, bed, plumbing and cycling, and home aquaponics helps you pick a size and spot for it.