caulerpa dying off......

[mahalamata]

i have had caulerpa in my fudge for the last 3 months.it grew to about 3 times the original bunch i put in but in the last week it has been receding.its looking bedragled and sorry for its self.i have a 9w compact light 24/7 lit over it,LR and LS in the fudge with it.has anyone experienced this before?any ideas as to what is going on?someone told me that perhaps some plant fertilizer would help(lack of various irons in the water)??
any advice or thoughts are welcome
thanks