There are questions about if it's the processed food or if it's the fact that it's floating food. You see, goldfish are bottom feeders, and their bodies aren't necessarily adapted to deal with the air that gets sucked in when a fish eats at the surface.
That being said, I loathe the cheap foods, and I usually use a high-quality food (like Omega One, which makes sinking goldfish pellets, meant to bypass the whole "eating at the surface thing") or make my own.
If you choose to make your own food, the important thing is to vary it constantly (it's also good to do this with purchased food). Find a method of binding that works for you (I use gelatin when I'm making my own food) and use various healthy ingredients for each batch. All of my home-made fish food has
spirulina in it. Other than that, I use fish, shrimp, spinach, zucchini, lettuce, and whatever else I have around that seems healthy.