usually, drastic changes causes STN from base up. [lowering of nutritions too fast]
did you add more than usually po4 remover ? or did you use more than needed GAC after sometime of not replacing it ?
either way, at this point, when it starts, all we can do is to test parameters, and keep them stable, and hope for the best. if indeed it is a po4 removal ISsue, some Amino Acid might help, but I would just leave it and concentrate on keeping parameters stable...
JMO