what I'm testing is if I can isolate the GB. In other words, stop all flow between FT & GB. If so, I can add stuff like fertilisers and pesticides to the GB without killing fish.
Don't know what to say. The fish should provide all the fertilizer you want with minor exceptions that people have already figured out how to add. Using a good fish feed helps with this. As far as the pesticides I'd be very careful what you add. Just because you add them when the system isn't connected doesn't mean they won't be around when you reconnect it. Things like this wind up in lake sediments all the time. I think beneficials are probably a better way to go but that's just me. I can see where splitting the system would have benefits in treating the fish with salt without harming the plants.