Flakes make my goldfish floaty, too. But I'm really glad that she's recovering well. My goldfish boys don't really like eating spinach that much either. I almost have to trick them into eating it. I hand-feed my guys so they are trained to come and eat from my fingers. I wave my hands over the tank like I'm getting ready to feed them at normal dinner time and instead of the normal dinner, I'll stick in a cube of spinach. Because they are so "excited" for dinner they scarf it up before they know what it is (kinda like tricking a kid into eating veggies).
I've also found that cucumber is another goldfish favorite. I chop it up into bite-sized cubes and then freeze it. They go made for the stuff. Every once in a while when I have an orange, I'll cut up a slice and feed it to them. Kale, red lettuce, and collard greens are other good things to try.
Since we know you have a floaty goldfish, you should avoid foods like broccoli and green beans.
I'm really glad I could help you through this! Hopefully you'll have little (not for long!) Mona around for another 10 years.