You don't. As far as I know they are a combination of filter feeder and light.....feeder???? Sounds stupid but they use the light as well is what I am trying to say. They will feed off of the little itty bitty waste that is in your tank, as well as using the light, kind of like a regular plant. My lfs recommended adding this black powder to help feed them. But they recommended ALOT of things to add to my tank. The way I look at it any unused black powder will just turn to waste in the tank.
So long story short, I add nothing special to my tank for my GSPs and they are doing just fine.
Scott