So I have some GSP growing on my rockwork, and I DO like it, but I planned a specifric pattern and area I wanted to grow it within. Well now (as GSP always does) it has started spreading to areas that I don't want it to be at. I tried peeling it away and also scraping at it with my nail (I wore gloves) but it didn't really do much other than make my entire colony close up and only a few pieces came off the rock. So I was wondering, is there any way to stop the spreading of GSP without killing the entire colony? Kind of like a mark-off line that it won't cross? I've heard lemon juice but I don't want to kill the colony, just a certain section. I also heard something about kalk paste? Would that work? What is it and where do I get it? I do plan to grow corals on that area of rockwork which is why I want to remove the GSP from that area, so I was also wondering if I can grow corals on an area I used kalk paste on?