If I ever had a doubt, today's training session convinced me that Mango really understands the iPod cueing system.
I upped the hoop behavior's difficulty to a chain: Follow my finger -> stop and check in at front of tank -> receive iPod cue as reinforcement for check-in -> perform 3 passes through the hoop in response to this one flash of the cue.
And she did it! Almost every time! Awesome!
I have also come to a better understanding of Mango's view of The Rules of Engagement. She does a few reps, then she wants a "coffee break" to putter around the gravel, just in case something yummy fell down there. I'm not fighting her on this. Substrate foraging is deeply innate in goldfish. If I tried to get rid of this behavior during training, everything else might break down. This is an example of what Bob and Marion Bailey called "The Misbehavior of Organisms": some behaviors are so instinctive that they can't be trained out of the animal. So I let Mango have her 5-10 second coffee breaks, then I ask her to check in and re-engage in training. What I might eventually try is actually timing the coffee break, then gradually shortening its duration.
But for now, I'm just happy with my little fishie's golden performance!