If it's your first hydro grow, it could be one of a thousand things. My suggestion would be to read over the thread that amsterdamned left. If you still arn't sure, check the sick plants section, there are a few good guides.
Other than that, i can't help you untill I get a little more information. What nutes are you using, and in what concentration? what is your ph? how about temps? how close are the plants from the light?
I'm not too experienced with your growing style, but generally speaking I have found that soil needs few nutes, if any. If you are adding alot of nutes, that's probably the problem.
Also, nearly always err on the side of too much nutes, as apposed to too few.
Hope that helps.