An old fish tank can be used to create a homemade hydroponic growing system, called the water culture system, in which a growing platform floats on top of nutrient water. For this project, use a sheet of 1-inch-thick polystyrene as the platform. An aquarium pump with tubing with a splitter and air stones from an aquarium store are also used. Aquarium filter floss, proper grow lighting and hydroponics nutrient solution are other materials for this project.
Cut the polystyrene to fit exactly into the old fish tank. The polystyrene will sit over the nutrient solution to keep it from evaporating, and you will cut holes into it for the plants to grow out of. Be sure to cut enough space for stem growth. The air tubes will be attached to the air stones and placed at the bottom of the tank. Choose long air tubes that will connect to the air stones at the bottom to the air pump outside the tank to oxygenate the water consistently.
Fill the old fish tank with the nutrient solution in water. Position the plants into the cut holes. To keep the plants in place, pack filter floss around the stems so that the roots dangle in the nutrient solution. Growing medium such as sawdust or perlite can be used around the filter floss, if needed. Use proper hydroponics grow lighting for the fish tank system. Plants such as lettuce or bok choy works best for this system.
Algae must be controlled in this kind of system. To prevent algae growth, be sure to shield the bottom portion of the fish tank from your light source. Light encourages algae growth and it also breaks down the nutrient solution. Place cardboard or aluminum foil around the fish tank. Use hydroponics fertilizer only for your fish tank system, as regular fertilizer works in soil only. Also, a pH testing kit should be used to maintain a neutral pH balance in the hydroponics system.