Am I beating a dead horse if I agree with everyone above: You need to feed it with hydro nutrients.
I will be a bit more specific though.
Urea (common nitrogen source in most household fertilizers) does not work in a hydroponic system, it requires soil to make the nitrates available to the plants.
In addition to providing the Macro nutrients (N-P-K) you will have to provide the Micro nutrients to a sterile solution such as perlite.
Lastly, remember one of the important things that plants get in well draining potting soil is... both water and oxygen. In hydroponics there are many ways to provide this balance; it sounds like right now you are just watering the plant and it comes out the bottom right? If so, you are providing plenty of oxygen, but when the plant gets larger this wont be enough water/solution to keep it watered and fed, so you will need to setup a more conventional system. I recommend researching "dutch buckets" if you want a "real/pure" hydroponic system, or check out my other thread ill link at the bottom if you want something with no maintenance.
And I strongly recommend making your own hydro nutrient as follows:
5 gallons of water
Materblend 4-18-38 - 12 grams
Calcium Nitrate - 12 grams
Magnesium Sulfate - 6 grams
The cost per gallon is $0.016 (one and a half pennies)
Check out this thread about my DIY hydroponic/soil grow box:
http://thehotpepper.com/topic/54589-diy-guide-how-to-make-off-grid-hydroponic-self-watering-container/