If you choose to build your own tanks and troughs with a pond liner, there are a few different materials for constructing the walls. Some materials will work well, when combined with partial in-ground excavation, to create deeper tanks for growing plants with larger roots or for fish production. Smaller, pre-made fiberglass and plastic tanks may need to be raised off the ground for a more convenient working height, while larger pre-made tanks need a gravel or concrete pad in soft or sandy soil.

Materials for Tank Walls

The materials you choose for building custom above-ground aquaponics tanks must be able to support the weight of the water. Water weighs 8.34 pounds per gallon, and a commercial aquaponics system typically contains 30,000 gallons or more among all the tanks. The higher you need to build to accommodate a large gallon size, the heavier the total amount of water you must support. Minimum wall height (or pond depth) is generally 18 to 24 inches, with dramatically deeper tank depths needed for some perennials. This is why digging into the ground to put some of that weight below the surface of the soil can help reduce the cost of making troughs that are stable enough for years of use.

Concrete – either stacked and mortared blocks, or poured in place with forms – is the strongest and most durable material for building the hydroponic trenches or tanks. It’s compatible with partially buried installations and works well with a liner. Don’t attempt to coat the concrete with gel coatings or pool paint, as these products usually aren’t food safe, cost more than quality pond liner, require a lot of labor to apply, and often fail within months of installation. Concrete is also the most expensive material for building custom tanks, especially poured concrete with rebar and mesh reinforcement. It’s often the only choice for very tall tanks for high water volume.

Thick plywood, 2x4 and 2x6 lumber, and some strong fasteners can create affordable and surprisingly long-lasting hydroponic troughs and tanks. Use pressure-treated wood so that the residual moisture and leaks won’t cause the wood to rot in a matter of months or a few years. As long as the liner remains intact, there’s no chance of leeching from the wood into the growing tanks. Some commercial plans call for using fiberglass epoxy instead of liner to seal and coat the inside of the wood tanks, but this is a costly and time-consuming way to create a large-scale set-up. Liner is much faster to install and costs less per square foot than epoxy, even when you include foam board insulation between the wood and liner. Round steel stakes or ¾-inch-wide rebar should be hammered into the ground every 16 inches along the outside of the wood frames to reinforce them, even if you frame them with dimensional lumber and strong screws or bolts.

Metal is rarely used for tank sides because it’s heavy, expensive, harder to anchor, and tends to corrode from the constant exposure to splashed and evaporated moisture. However, some pre-fabricated systems are available that rely on interlocking metal panels that are paired with a liner. The edges of the metal sheeting used for the sides must be rolled or otherwise reinforced to prevent it from tearing through the liner. The material should also be powder coated, or otherwise sealed, to prevent corrosion from weakening the sides. Finally, it’s very hard to maintain a constant water temperature in metal-sided tanks, as the material is too conductive and transfers heat in and out easily. Skip metal tanks unless they’re your only option and you don’t mind insulating them and paying more for water heating in temperate climates.

Because nutrient film technique (NFT) aquaponics systems don’t rely on tanks or troughs for the hydroponics portion, the pipes or tubes used for that design require tables or sawhorse style supports instead. They’re relatively easily supported by concrete block or lumber stands, as only a limited amount of water flows through the pipes at any given time.

Digging In-Ground Troughs and Ponds

After working up figures for the cost of concrete or wood sides for your hydroponic tanks, you may be eager to cut those numbers somehow. If you’re working with relatively level raw land, you can likely dig down at least a few feet to put some, most, or all the tank’s storage area below the surface. This transfers the work of supporting thousands of pounds of water to the soil rather than pressing against the walls you construct. Other benefits of in-ground aquaponics ponds and trenches include:

• Natural insulation from the soil; reducing heating costs and preventing fish losses due to sudden fluctuations in temperature.
• Easy installation of additional insulation; most commonly used in the form of solid foam boards cut to fit the contours and shapes of the trench or pond, which also reduces the amount of smoothing necessary before you install the liner. (Insert Image IMG_0839.jpg)
• Sizing is the same; with relatively little water volume lost to the gradually sloped sides of an in-ground trench compared to the sharper edges of an above-ground tank.
• No need to replace the material used for the walls every few years or decades; only the pond liner as needed.

It is a little harder to find a leak when one occurs in an in-ground pond, but because the water tends to escape into the soil, you also aren’t left dealing with erosion or flooding when a leak occurs. To minimize the chances of leaks, take extra care when digging the trench or pond to smooth the surface as much as possible. Remove all rocks and even small stones and pebbles, as they will press against the liner and create a point of wear. Aim for gradual slopes and smooth transitions between the bottom and sides of the trench. Most importantly, select a high-quality liner for reduced chances of leaks and wear during the life of the pond.