Sometimes you want to grow plants without using "media." In that case, you need some method for suspending your plant over the surface of the water.
I've seen folks use foam rafts, wooden structures supported on air-filled water bottles, or water-filled bins with holes cut in the lid. The plants are put in "net pots" to provide some support while allowing the roots to grow through the mesh/net of the pots.
I've used foam rafts. But you want to make sure you select the 'right' foam. There are three basic foam insulation board products on the market produced under several different manufacturer names. The basic types of foam board insulation include: polystyrene, polyurethane or polyisocyanurate.
- Expanded Polystyrene (EPS) - This is the white foam board that is made up of lots of tiny white foam balls. Even though this isn't toxic, you don't want to use this for aquaponics. The stuff will disintegrate relatively quickly, leaving you with a white, snowy mess.
- Extruded Polystyrene (XPS) - This is what you want for aquaponics. Folks typically recommend the 2 inch thick Dow product (blue board). However you may find other brands at your local hardware store, and the colors will vary depending on the manufacturer. For example, pink is the trademark color for the Owens Corning product, Lowes' GreenGuard product is (surprise) green. In all cases, make sure you're dealing with extruded polystyrene rather than some other chemical.
- Polyurethane (PUR) or Polyisocyanurate (PIR or ISO) - Fully reacted polyurethane, itself, is chemically inert. The problem is that the stuff you can buy at the hardware store is typically "improved" to increase the insulation properties (R value). This "improvement" is achieved by adding isocyanurates, which are hazardous chemicals. The list of potential human hazards from exposure to isocyanurates is long and scary - and I wouldn't want my fish exposed to this stuff (or have the roots of my plants bathing in a weak isocyanurate solution). Folks on the web will often equate pink foam with polyisocyanurate, but that is not necessarily the case. Check the chemical composition of the foam you intend to buy and just make sure you're getting XPS and not PIR or ISO.
Lay out where the centers of your holes need to be. I use 3" spacing between centers for the 2" net pots I'll use to start seedlings or allow cuttings to root. You'll want to use 6 inches or more between centers if you want your plants to grow to maturity in the floating raft - this will be OK for plants like lettuce.
To drill the holes for 2" net pots, I recommend a standard 2-1/8 inch Door Knob Hole Saw and another drill bit that is long enough to go all the way through your foam. Drill a small hole at the center of each location, then drill part through one side at each location with the hole saw. Flip the foam and finish drilling the holes from the other side, using the center holes as a guide.
To prolong the life of your raft, you can paint the foam with white latex paint.
Here's a video showing the basics of a 2 inch thick floating raft that will fit in a 50-gallon Rubbermaid tank.