Do you want to understand the drivers for fish stocking densities in aquaponics systems? Then start by recognizing that maximum levels are gated by how much feed can be ‘processed’ by the fish and then how it is filtered from the fish environment as solid waste. The more sophisticated the filtration system, the more densely the fish can be stocked. This leads to point two – media-based systems use the grow media itself as a mechanical trap for the solid waste while Deep Water Culture (DWC) or raft-based systems use solids removal equipment that is specifically designed to capture those solid wastes so the system operator can remove them.
Today’s DWC aquaponics systems evolved from the good work of Dr. James Rakocy and his team at the University of the Virgin Islands (UVI). They approached aquaponics from an aquaculturalist’s perspective, asking and answering the question ‘how many fish can I grow and still grow plants productively?’. Because their focus was primarily on fish system productivity, they employed sophisticated waste removal techniques, thereby creating systems that can handle high levels of solid fish waste. DWC systems with solid waste removal equipment can generally handle up to one pound of fish for every two to three gallons of water IF they are designed properly and well maintained.
There are aquaponics systems vendors that have created turn-key DWC systems and plans for building your own DWC systems that employ the UVI methods. Many of them work pretty well, but they tend to be much more expensive to buy or build than media based systems because of the need for the extra solid-waste removal components. In addition, they require daily maintenance levels that are far beyond that of media-based systems because those solids filters need to be cleaned regularly and the fish poop handled, removed and properly disposed. In fact, one DWC systems vendor claims to be able to grow 200 pounds of fish in their small system which has four 50-gallon fish tanks. While I would love to see actual data supporting this incredible level of productivity in such very small tanks, I give them credit for at least acknowledging that in order to have a chance at achieving this you would need to spend approximately an hour a day maintaining such a system. So it comes down to a large up-front investment (in this case $6895 for 200 gallons of fish tank and 72 square feet of growing area) and 365 hours of labor a year for 200 pounds of fish and large quantities of leafy greens and herbs (the plant types that DWC systems grow extremely well).
A media based system, on the other hand, requires no external solids filtration. Instead, the solid waste is sent straight into the grow beds where it is mineralized (broken down into a form that is available to the plants, thereby adding to the food available to the plants) by heterotrophic bacteria and composting worms. Because this is a natural, balanced system versus a more human-intensive system; stocking density recommendations tend to be a more conservative one pound of fish for every five to ten gallons of water.
The good news about this lower level of fish production, however, is that you can eliminate the expense, space requirements, and complication of the solid waste removal components from your system (a clarifying tank, mineralization tank and degassing tank) and just install a fish tank, grow beds, and a sump tank if needed. In fact, our AquaBundance Modular Bountiful 6-bed System has the same 200 gallons of fish growing capacity and the same 72 square feet of plant growing area, but it only costs $4095, a $2800 saving over the DWC system mentioned above. Plus, because the bacteria and the worms are doing the clean-up work for you, maintenance drops to an average of about five to ten minutes per day. This time goes primarily to feeding the fish, occasionally rinsing the pump intake, topping off tanks as needed, and working with the plants.
Let’s look at this as an economic equation. What if you value your time at $20 / hour, for example? Compare the 200 gallon DWC system at (365 days * 1 hour/day * $20) / 200 pounds of fish per year = $ 36.50 / lb of fish and accompanying vegetable output versus a media based system with a 200 gallon tank at (365 days* 1/9 hour per day * $20) / 40 pounds of fish per year = $20.28 /lb of fish and accompanying veggies. And the economic news gets even better with a media based system because they cost less to begin with. Plus, while media based systems might not be as efficient at producing lettuce as a DWC system, the higher levels of nutrients available to the plants through the mineralized solids wastes along with the structural support that the media offers the plants makes them far better at producing higher value fruiting crops like tomatoes, peppers, eggplant, strawberries, melons, squash, etc., etc. And that decomposing solid waste actually acts as a ‘nutrient bank’ in media based grow beds that allows you to weather fluctuations in fish stocking levels without starving your plants.
The choice is yours. If maximum fish production is your most important buyer value regardless of how you price your time or how dear the dollars needed for your initial system purchase are to you, then you should be considering a DWC system. But if you are looking for a balance between high levels of fish production and high levels of vegetable production with the lowest possible maintenance and initial setup cost then you should be considering a media based system. And if you are looking for the best aquaponics systems available in the U.S. today, you have no further to look than our AquaBundance Aquaponics Systems.