You're right, the forage pond is naturally very fertile. Why that is, while the main pond isn't, is a mystery.
I feed AquaMax MVP to the adult BG. Not much at first, since there are so few, but gradually ramp it up as the fry get larger. I'll mix in some smaller feed as the summer goes on.
As for your forage pond, perhaps you should stock some fathead minnows a couple of months ahead of the BG. A few wooden pallets would make good spawning substrate for them. Wish I'd done that last fall, but you can't think of everything.