Good question. I would be interested in hearing inputs on that as well. I have a small school of GSH that seems to never decrease much or increase much from year to year. Although this fall the school was slightly larger and I could see what appeared to be some SFS had joined the GSH school.
My GSH got started when I found a few (3 or 4) hitchhikers mixed in with my original FHM stockers and went ahead and tossed them in. So my gene pool has to be weak! I'm considering boosting the GSH numbers a good bit as I've recently read in some old threads that they will eat a lot of FA if other pond species make zooplankton, etc. scarce and force them to FA. In my case, I'd like to know how many to stock in a pond that already has apex predators.
I'd also appreciate confirmation that GSH eat FA if anybody has experience/knowledge they could share