Now, let’s look at the basics of deep water culture (DWC), which is also commonly called raft culture or cultivation. Put simply, build a big tank, trench or pond and float some lightweight foam or Styrofoam rafts on it. Punch holes in those rafts, tuck plants in the holes with something to anchor their roots, keep the water quality balanced and watch the floating plants grow rapidly.

For most commercial growing operations, DWC is the method that will produce the most profits. The system allows for a much higher stocking rate of fish and therefore requires an extensive growing space for plants. It is possible to maintain higher nutrient levels with DWC without risking plant damage due to the water volume in each tank or trough. Therefore, allowing you to grow heavy feeders like tomatoes and fruit bushes that are considered impossible, or unfeasible, in other systems.

In addition to building trenches or troughs that are at least 18 to 24 inches tall or deep, you’ll need to size them to your projected raft size. The guidelines provided in this section will help you determine the amount of surface area you will need for growing space in relation to the amount of fish in your system. Once you know that raw number, you can split that total surface area up into different shapes and sizes to create your rafts. Almost all DWC systems are built around two-foot-wide rafts so workers can reach plants in the middle without straining, and two- to-four-foot lengths keep individual rafts from being too heavy to lift.

Design your trench dimensions around those rafts; leaving at least one inch of space on either side, so the foam doesn’t fit tightly into the surface of the tank. Tight-fitting rafts prevent algae growth, but they’re also very difficult to remove at harvest time or to lift periodically to inspect the roots. Most aquaponics-safe foam tends to crumble under your fingers, so, a gap on both sides is recommended. This allows for easier handling without damage to the rafts, which you will want to use repeatedly.

Use only pre-made rafts, sold by aquaponics suppliers, food grade bead Styrofoam (used to make coffee cups), or DOW brand Blue insulation board for the rafts. There are pink and yellow insulation boards sold by companies like DOW and Owen-Cornings, but these are made with polyisocyanurate. Polyisocyanurate can leech chemicals into the water, that hurt or damage the fish and plants, even when coated. If you’re making your own rafts rather than purchasing them, you’ll need to cut holes for the net pots or other growing medium. Net pots are recommended for commercial use because there’s no need to fill them and they’re reusable. Find the recommended spacing for your plants on the seed packet and draw out the 2-inch circles on a sheet of plywood first. Cut the holes out of the plywood and then use it as a stencil for rapid creation of rafts that are all uniform in size and layout.

After cutting the holes, and before installing any cups or net pots, you’ll need to paint the top surface of the material. This prevents it from degrading rapidly when exposed to sunlight; as UV-stabilized products are often unsafe to use in aquaculture. Use a white exterior latex paint that is confirmed by the manufacturer to not contain any mildewcide ingredients. This is important because these elements are toxic to the bacteria, fish and plants in your system. Don’t just assume a paint is safe to use because the label doesn’t list any mildewcides. Because formulas change constantly without warning, you must call the manufacturer every time you plan to paint new rafts to find out which paint from their product lines is safe to use for this application.

Benefits of DWC:

• The growing material in cups or the medialess net pots suspended in each raft can act as the biofilter for the system; eliminating a separate tank or media bed.
• The higher volume of total water compared to NFT creates a buffer against sudden changes in temperature, dissolved oxygen or nutrients around the plant roots that could kill them.
• Easy harvesting; by lifting up the individual rafts and moving them to tables or sawhorses for standing work with no bending or twisting.
• Eliminates the need for flushing or draining; except in rare circumstances where extensive cleaning is needed. Most solid waste that does make it to the grow beds is easily scooped or vacuumed out after lifting the rafts out of the way.
• Gravity flow between the fish tanks, intermediary filter tanks and grow beds allow for the use of just a single sump pump to return water back to the start of the system.
• Lower upfront costs to construct, reduced labor costs for long-term maintenance, and relatively easy repairs or replacement in the long run.
• More even light distribution; as the large trenches or tanks can’t be stacked or packed too tightly together like other systems.
• No growing medium in the water; prevents solid waste from becoming trapped. Reduces clogging and anaerobic bacteria growth, which occurs in media-filled beds, resulting in less extensive cleaning.
• Much easier to apply treatments for diseases and pests because the plants are growing in dense blocks rather than long strips of individual plants in a row, as with NFT systems.