Whenever I have a sick plant my hydro guy tells me to cut my ppms in half......the plants must have SOME nutes available in order to get well but go light until they are looking happier.

Stinky may have an excellent point with the salty crust on your hydroton.....might be a wise idea to flush the system to prevent salt build up etc.....salt/fert buildup in your medium can cause almost every issue out there.