I have the same problem with GSP in my sps tank. I had reacquired a frag from my other mother colon of GSP. It one of the very first coral I ever bought. New the tank was young it did fine and grew fast. As the tank matured and the sps started to take off the GSP slowly died off. I think it is because GPS like a "dirtier" tank. I have some GSP in my 6 gallon nano that is skimmerless and they do great.