If its a N toxicity then your ppms will slowly be increasing, your have yet to list your ppm numbers. As long as your ppms slowly drop, you wont have a toxicity if your following the flora chart at half strength.
What does those roots look like? If they are fine, and your ppm is dropping, its the light being too close as the others mentioned. If you cannot raise it, defoliate alot of the upper fan leafs to allow light down below.
I would try to pull them down too, and posibly remove some of the lower sites.
