Might be worth trying an ICP test. There are a number of things that can cause GSP to be unhappy. Anything from REALLY high Phosphate, Alk problems, salinity instabilities or even temperature problems.
What temperature is your tank, and what do you use to check it?
Also what is your salinity/SG and how do you check that?
If neither of those, an ICP can let you know if something is out of whack or maybe something is rusting (high iron) etc.