I thought the issue with flake is that it contains so little moisture, when its swallowed it begins to absorb moisture and expand inside the gut leading to bloating. I find flakes are ok if you crush them them up when feeding, almost to a powder. It gives them a much larger surface area so they absorb water almost instantly.
I know with corydoras that they actively gulp air (aerial respiration), which actually runs through their gut, and is later farted out ? Maybe other fish struggle if they gulp some air? Seems like a bit of a pathetic issue for an organism thats been around for hundreds of millions of years to have? If they gulp a bit of air, their swim bladder gets bloated, they struggle to swim and end up dead?