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Cooler Float Grower
We feed the seeds with plain water until the plants come up and then switch to a dilute vegetative growth solution (inorganic powder; calcium nitrate, magnesium sulfate, and a "grow" mixture). We usually use Corvallis municipal tap water to mix our nutrient and haven’t had any problems with nutrient imbalances or toxicity. When the young seedlings are ready, we cut the cells apart, cut the bottom off each cell (make sure your scissors have been sterilized), gently shake out the media (it falls right out), and insert the cells into the cooler lids. The cells fit snugly into the holes and the plant roots easily dangle into the nutrient solution from the start. Oasis or rockwool propagation cubes would probably work for this system as well. The plants should be large enough so the bottom leaves of the plant will support it so it doesn’t slip into the cooler. We cut a hole into the side of the cooler near the top and to accommodate an air tube connected to an air stone at the bottom of the cooler. An aquarium pump provides constant, gentle, bubbling oxygen. We spilt the air line coming out of the pump and run air tubes to both coolers. One 3-watt pump ably provides oxygen for two coolers. Both coolers easily fit under one of our 250-watt metal halide Hydrofarm Sunburst lights, which is connected to a timer set for a 12-hour photoperiod. Three or four coolers--or a few larger coolers--could probably fit under such a light if reflectors were used. The light is positioned about one foot or more from the plants to make sure they aren’t exposed to too much radiant heat. Most salad greens and lettuce plants go from seed to harvest in just over a month. If you selectively harvest, once the plants are maturing, you can pull enough greens off for a few salads a day for a few weeks. Don’t wait too long to harvest since overly mature greens can get bitter. Greens will stay nice and fresh in the refrigerator for about a week. If you harvest all at once, you’ll have enough greens for a few mighty salads. If you have new starts continually going and regularly replace old plants with new, you can always have enough greens for the table. The cost of the electricity to run the light and pump (and the cost of the nutrient, which is insignificant) works out to be much less expensive than what you would pay for the greens in the store (about 1/2 as much). And it’s much more fun to grow your own pesticide-free food.
Some similar system instructions, as conceptualized by Wally Oi and Charlie Musgrove in Hawaii (they have taught students how to build and operate hydroponic systems on the islands), are on The Growing Edge Web Site (see http://www.growingedge.com/basics/easyplans/ezegro.html). Materials Styrofoam cooler, seed-starting trays, seeds, utility knife, scissors, aquarium pump and tubing, air stone, HID lamp (optional), growing medium, nutrient
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In the early days of winter 2001-02, we made some new floating systems out of those "disposable" Styrofoam coolers. Each one costs about $3 new but people are always throwing these coolers away during the summer. If you find one for free, just make sure to give it a good cleaning before using it for a hydroponic system. The coolers we’re currently using are tapered--14 inches long at the top and 12 inches at the bottom. The depth is 8 inches. Since the white Styrofoam will very gradually permit liquids to seep out, we lined the inside of the cooler with two layers of strong plastic, secured with duct tape along the outside rim. We flipped the lids over and cut six spaces on each lid to accommodate the plants. The lids are domed, so when they’re flipped over, they drop down a few inches below the top rim of the cooler. We started our plants in those flimsy black plastic cell trays found at garden centers everywhere (each cell is 2 inches square and trays usually have around 12 cells apiece) in a loose mixture of 
For salad greens, we run a low conductivity solution (an EC of around 0.5-1) and every so often replenish with either water or 