i think it is overwteringIts hard to tell without a pic. BUT, it sounds like over warering. How often do you water? With seedlings it is safe to go every three to four days. Over watering will cause your leaves to curl/yellow/brown and die. If it is overwatering hopefully you haven't had root rot set in.
i run 24 on lightsNormally, if you see burning and you have a decent pH level the lights are too close. Move your lights back at least 10" at first and then try moving them closer. I don't know what light cycle you are running either but try 18/6 if you're using 24 on. The only thing else is water and keep regulating the pH.