Choosing and Sizing a Fish Tank for Your Aquaponics System

The fish tank is the engine room of an aquaponics system. Get it right and everything downstream, from water quality to plant nutrition, becomes easier to manage. Get it wrong and you are fighting problems that trace back to the tank itself. Before you buy or repurpose a tank, it is worth understanding what shape, depth, volume and material actually mean for your fish and your system as a whole.

Why tank shape affects water quality

According to Wilson Lennard's aquaponic fact sheet series (Aquaponic Solutions, 2012), aquacultural science has established that round or circular tanks provide the best fluid dynamics of any tank shape. The reason is straightforward: a round tank has no corners in the vertical dimension, so water flows in and out without obstruction. Corners in rectangular or square tanks can slow or accelerate flow in particular regions, creating areas where water is not properly exchanged. Lennard (2012) describes these as "dead zones" where solids can accumulate, dissolved ammonia can build up, and dissolved oxygen can drop.

Three things depend on water moving correctly through a fish tank. First, fish waste solids need to be carried out of the tank promptly. Second, dissolved ammonia needs to reach the biofilter quickly enough to be converted to nitrate before it reaches harmful concentrations. Third, dissolved oxygen needs to be maintained throughout the entire water volume. If corners are trapping pockets of stale water, none of these three things can happen reliably.

For hobby-scale systems with low stocking densities, Lennard (2012) notes that the consequences of a non-round tank are less severe, because the low fish load means water quality problems develop more slowly. Even so, the same source makes the point clearly: hobby-scale practitioners should not completely ignore the principles of fish tank design. If you have the choice, a round or oval tank is always preferable to a square or rectangular one.

A practical implication of round tanks is where to place your pump or outlet. In a round tank, solids settle predictably toward the centre of the floor, and the pump or drain can be positioned there to remove them efficiently. Lennard (2012) notes that in non-circular tanks it is far more difficult to predict where solids will settle, which makes outlet placement much less reliable.

Surface area to volume ratio and gas exchange

Tank shape also affects how well the water body exchanges gases with the air above it. Lennard (2012) explains that the majority of oxygen uptake and carbon dioxide release in a fish tank happens at the water surface. If the surface area is too small relative to the water volume, gas exchange can be restricted to the point where oxygen cannot enter fast enough and carbon dioxide cannot escape fast enough.

The practical consequence is that deep, narrow tanks are worse for fish than shallower, wider ones of the same volume. Lennard (2012) recommends a minimum surface area to volume ratio of 1.0, where surface area is measured in square metres and volume in cubic metres. As a worked example from the same source: a round tank holding 2,000 litres with a 2 metre diameter and 1 metre depth has a surface area of approximately 3.14 square metres, giving a ratio of about 1.57, which comfortably exceeds the minimum.

For backyard systems, this points away from tall, narrow vessels (such as upright water storage tanks used without modification) and toward wider, lower-profile tanks where the water surface can do its job. If you are considering a rainwater tank, the common approach described in Practical Aquaponics for Everyone (2007) is to cut the top out of the tank and use it on its side or as a standard open-top vessel, which improves both access and gas exchange.

Minimum water depth

Practical Aquaponics for Everyone (2007) makes a simple but important point about depth: tanks that are too shallow are not conducive to fish happiness. The same source recommends a water depth of at least 750 mm. Fish need a reasonable column of water to move through, and shallow tanks also tend to experience greater temperature swings, particularly in climates with strong sun. In the wet tropics and other subtropical regions, a tank sitting in partial sun can heat up quickly in a shallow vessel, stressing fish that prefer stable temperatures.

This 750 mm depth recommendation does not conflict with the surface area guidance above. The goal is a tank that is wide enough to have an adequate surface area to volume ratio, but also deep enough to give the fish comfortable living conditions. A broad, squat tank with 750 to 900 mm of water depth and a generous surface area satisfies both requirements.

What volume do you actually need

Volume is tied directly to how many fish you want to keep and at what stocking density. Lennard (2012) notes that commercial recirculating aquaculture systems run fish at densities up to around 50 kg per cubic metre in aerated systems, and higher still with direct oxygen injection. Hobby-scale aquaponics practitioners rarely approach those densities; most keep fish well below 20 kg per cubic metre.

Practical Aquaponics for Everyone (vol. 2) frames stocking density in units that are easier to work with at home: kilograms of fish per 100 litres of water. The same source lists the factors that determine how much fish a tank can carry: tank size, fish size and species, dissolved oxygen levels, and the biofiltration capacity of the grow beds. These factors are all connected. A larger tank gives you more water volume to buffer changes in water quality, more surface area for gas exchange, and more room for fish to behave normally.

For someone starting out, Practical Aquaponics for Everyone (2007) recommends beginning with a small system, citing a first system built around a 230 litre tank. The same source strongly recommends starting small and scaling up once you have learned how the system behaves, rather than buying a large tank immediately. A 1,000 litre IBC (intermediate bulk container), which measures roughly 1 m x 1 m x 1 m, is described in the same publication as an ideal size for a domestic aquaponics fish tank. It is large enough to give reasonable buffering capacity while still being manageable for a first-time operator.

The volume of your fish tank also needs to be matched to the volume of your grow beds. The ATTRA aquaponics guide notes that early systems used a 1:1 ratio of fish tank water volume to hydroponic grow bed media volume, but a 1:2 ratio is now common, and ratios as high as 1:4 are used. The appropriate ratio depends on fish species, stocking density, feeding rate, and the type of grow bed. The lesson from Kevin Cuthbert's system, described in Backyard Aquaponics Magazine (Issue 2, 2008), is a cautionary one: starting with 5,800 litres of water and only 1,260 litres of gravel left the grow beds unable to process the ammonia load from a heavy fish stock. The grow bed volume was a small fraction of what the fish required, and the system suffered badly as a result.

Tank materials

Lennard (2012) identifies fibreglass and polyethylene (PE) as the two safest and most practical materials for fish tanks. Both are available as pre-formed tanks, both are widely used, and once properly cured and hardened, neither tends to leach compounds that affect fish. Most purpose-built fish tanks sold for aquaculture and aquaponics are made from one of these two materials.

Other options are workable but come with caveats. Stainless steel can be used, as can PE liner inside a structural shell. Practical Aquaponics for Everyone (2009) notes that fibreglass is strong, stable, and once fully cured is chemically inert, making it well suited to aquaponics. The same source also mentions using a previously stored 250 litre fibreglass tank that was cleaned and recommissioned for a nursery tank, which illustrates that second-hand fibreglass tanks are a reasonable option provided they are in sound condition and have been cured.

IBCs are widely used in Australian backyard systems. Practical Aquaponics for Everyone (2007) notes that IBCs in good condition can often be sourced second-hand. The important check with any second-hand IBC is what it previously contained. Containers that held food-grade liquids are generally safe after thorough cleaning; those that held chemicals or industrial fluids are not suitable for fish, regardless of how well they are cleaned. Check the IBC's history before committing to it.

Poly rainwater tanks are another practical option. Many people cut the top out of a suitably sized rainwater tank to create an open-top fish tank, as described in Practical Aquaponics for Everyone (2007). Poly cattle troughs, where available, can also be suitable. The key questions are whether the material is food-safe, whether the dimensions give adequate depth and a good surface area to volume ratio, and whether the tank can be positioned and supported securely on level ground.

Practical decisions before you buy

Working through a few questions in order will help you narrow down your choices. Start with where the tank will go. The site needs to be flat and level, because a large water-filled tank exerts enormous weight and any unevenness will stress the tank walls. Sun on the fish tank encourages algae growth, so a location with shade on the tank but good light for the grow beds is preferable. The IBC of Aquaponics guide (Backyard Aquaponics) recommends keeping sun off the fish tank as much as possible.

Once you have a site, work out the maximum footprint and height you can accommodate. From that, calculate the volume. Use the 750 mm minimum depth from Practical Aquaponics for Everyone (2007) as your floor, and check that the resulting surface area gives you a ratio of at least 1.0 against the water volume, as Lennard (2012) recommends. Then decide on a target stocking density well below 20 kg per cubic metre to give yourself a comfortable margin while you learn the system. Multiply your target fish weight by that density to confirm the tank volume you need can carry the fish load you are planning for, and cross-check that your grow bed capacity will be at least double the fish tank water volume.

If round tanks are not available or practical for your situation, a round or oval shape should be the aspiration rather than a fixed requirement at hobby scale. What matters most is that you avoid very tall narrow vessels, maintain adequate depth, choose food-safe materials, and size the tank so that your fish have room to grow without the water quality becoming a constant struggle.