Must be the GBA's bro, your gunna have to give them too me
Nah but seriously, Possibly something to do with double dosing the ferts? I found that when ever i overdosed the PMDD my tank would go green, and alot of the stem plants would die off/not look too good. If I was you, i would just do a water change, then keep dosing the correct amount of ferts until you see some improvements... What kind of bubble rate do you have coming from the co2? With 300W's over a 200-250L (i think yours is about that isnt it?) You will need a highish bubble rate (1 bubble per second at least)
Whenever things like this happen to me, i just chuck alot of the dead/dying/not looking good plants out and re-plant the whole tank with cuttings, and re-do the tank again pretty much.
HTH