Hydroponic systems provide water and nutrients to plants without the use of soil as a medium. If you're growing tomato plants using a hydroponic system indoors and the plants do not produce fruit after flowering, it may be due to a lack of pollination. Outdoor plants are pollinated through wind, bees and other means of transference. Indoor plants may need your assistance in order to complete the pollination process and produce tomatoes.
Place oscillating fans in the area with your plants. Create the effect of wind using fans. Place the fans at opposite ends of the room so that the air movement crosses through the tomato plants.
Gently shake the tomato plant. Tomato plants depend on motion to activate pollination. You can simulate the motion of wind by vibrating your plants. Hold the plant by the main stem just above the middle. Gently shake the plant back and forth. Repeat for all your plants.
Use an electric toothbrush. Hold the toothbrush on the stem near the base of the flower. Turn the toothbrush on and allow the stem to vibrate for several seconds. Repeat with each flowering stem.
Note that tomato plants are self-pollinating. This means that each flower is both male and female. The simulation of wind through fans and vibration "shakes up" the interior of the flower and pollination occurs. Moving pollen from flower to flower simulates the activity of bees and other insects. If you are growing different varieties of tomato plants, wind simulation may result in cross-pollination.
Hand transfer pollen using a small brush. Using a brush for pollination reduces the chances of cross-pollination. Place the tip of a small artist's paintbrush into the flower and move it slightly until you see pollen clinging to the bristles. Carefully pull the brush away from the flower and place the tip, now covered in pollen, into a neighboring flower. Gently shake the tip of the brush in the flower. The brush will deposit pollen and pick up fresh pollen, just as a bee would. Move from flower to flower with your brush.
Set up your hydroponic systems outdoors. Take advantage of the natural growing season in your region and set up your hydroponic tomato plants outside. Their exposure to the wind and bees will be the same as if the tomato plants were in a soil garden. The plants then will pollinate naturally.