Here is a thread with a bunch of pictures of RESxGSF hybrids and potential hybrids to help with identification.
GSF always have a translucent orangish or whitish border on their opercular tab. Some are more prominent that others. At times in the past I think I have mistaken a GSF for a hybrid with some RES in it because the particular GSF just had a more pronounced orangish border.
What I find in the ones I am sure are hybrid RESxGSF (because they came from a pond with no BG) is 1. the border is darker and more solid rather than translucent and 2. often is "broken" rather than being a consistent color around the entire border. You can see some very good examples of this on page 6 of the thread.
RESxGSF hybrid thread I think they are really pretty fish. I had a forage pond with mostly RES but a few GSF and created loads of RESxGSF hybrids from that pond. I really like them.
If you have a BG with anything other than a "solid" ear tab, it is a hybrid of some persuasion.
Good luck on your BG management.
