Sizing a System for Deep Water Production

Deep water culture (DWC), also commonly called raft production, is the most popular form of commercial aquaponics in warm climates. This system uses a lot of water for the hydroponics portion of the equipment, so most systems using rafts have a ratio of about seven times as much water for plants as for the fish. In the University of Virgin Islands system, the original commercial raft aquaponics design (four 2,000-gallon fish tanks) is paired with four 3,000-gallon troughs. Add in a little over 2,500 gallons worth of sump and settling tanks and you have a 22,500-gallon system that produces up to 4.75 metric tons of tilapia and five metric tons of basil in a year. Raft systems like the UVI design tend to feature higher than usual fish stocking rates and extra tanks and filters to handle the heavy load of dissolved solids.

Ideal feed ratios for DWC range from 60 to 100 grams per square meter of growing area per day. This produces the right balance of nutrients without overwhelming the filters or leaving the plants starving for nitrogen. This range is relatively wide to accommodate the needs of different stocked fish. Assuming a daily feed ratio of 75 grams/m2/day to keep tilapia growing rapidly, the following figures would be possible:

• Average daily feed rates of 150 grams per 100 tilapia
• Stocking rate of 2 lbs. of mature fish weight per 1 cubic foot of rearing space
• 1,000 lbs. of fish in two 2,000-gallon tanks
• 1,500 grams of feed per day
• 1,500 grams/day divided by 75 grams = 20 square meters of growing space required

While 1,000 fish may sound like a lot, they’re only supporting a relatively small growing area at that high stocking rate. If you want 300 square meters of growing area at a daily feed ratio of 100 grams/m2/day, you’ll need around 20,000 lbs. of tilapia to support that ratio. At a 2:1 ratio of pounds of mature fish to cubic feet of water, you’ll need over 75,000 gallons of water for all those fish. Some deep water culture systems run on daily feed ratios as low as 10-20 grams/m2/day, but those rates lead to such slow growth that the idea is rarely viable for commercial production. Plan for a minimum ratio of 50 grams/m2/day minimum for raft production that is actually profitable.

Sizing Aquaponics for a Nutrient Film Technique Installation

With the greatly reduced volume of water required for flushing the plant roots inside the pipes of a nutrient film technique (NFT) system, it’s no wonder that daily feed ratios are about 25% of those recommended for DWC/raft culture. Too high of a stocking density or oversized tanks, resulting in too much feed entering the system, will lead to nutrient overloading and damaged or dead plants. Assuming proper filtration and settling tank additions among the fish and plant tanks, NFT systems should have a daily feed ratio from 15-25 grams/m2/day. With a chosen daily feed ratio of 20 grams/m2/day:

• Average daily feed rates of 150 grams per 100 tilapia
• Stocking rates of 1 lb. of mature fish weight per 1 cubic foot of rearing space
• 500 lbs. of fish in two 2,000-gallon tanks
• 750 grams of feed per day
• 750 grams/day divided by 20 grams = 37.5 m2 of growing space required

While these low stocking rates and daily feed ratios can save money on fish feed supplies and filtration units, they also result in relatively small harvests of fish compared to DWC systems. Unless only plants are profitable in your market and you are only involving fish in the system as a fertilizer source, raft culture will likely result in higher profits than NFT growing methods for the low stocking densities and daily feed ratios alone.

Sizing for Pond-based and In-ground Systems

Pond-based aquaponics are generally used for DWC/raft production, so you can use the same numbers and feed ratios as you would for above-ground troughs. If you use in-ground media beds paired with in-ground ponds for fish production, you’ll need a slightly different sizing plan. You’ll need to use a calculator to work out the size of the system according to the desired size of the bed and whether you will treat them as the biofilters or add other filtration. With fewer filters and tanks to capture solid waste, it’s also important to adjust the calculations based on the protein level of the feed. There are too many variables to consider to simply provide a formula, but you can find multiple free and reliable media bed sizing calculators online.

Sizing for Non-Recirculating Decoupled Systems

Decoupled aquaponics systems are rare and still in development for commercial use. There is only limited data available on which to base designs and measurements for these systems. So, most growers interested in using them must experiment on their own to find ideal sizing and daily feed ratio numbers. Even in the systems developed by private companies and engineers, the sizing requirements are so complex they require proprietary consultation from the system developers.