Suggestion: I don't know if you have already tried this method when feeding flakes. When I had community tanks to get the flakes to the bottom of the tank when flakes where on the menu for that feeding, I used a cup and put some tank water in it. I soaked the flakes for about 15 or 20 minutes. This makes them expand by getting the air out because they do contain a certain amount of air. After they have soaked, I slowly poured the cup containing the flakes into the tank. They don't sink quickly and all fish get a chance to eat flakes even the bottom feeders. Now, when feeding flakes to the one Betta only that I have, I soak one or two flakes depending upon the size of the flake. This gets the air out and it will make the flake dense because of the absorbtion of water. Not as dense as a pellet but not as light as a flake right out of the container. I have not had a problem with any fish getting bloat from flakes by using this method.
This doesn't seem like it would be good for the water quality.
Good point...now that I see it in print. It could depending upon the brand of flake used. I have never seen an oily sheen or film on top of the water on any of the tanks previously owned. Now that I see what you mean in print right in front of me, I am thinking back on Brand labels and I think that is entirely possible.
Again..I have not seen any oily sheen or oil residue residing on the water surface during and/or after feeding. This is what is so great and wonderful about hobbies, in general and hobby forums, in particular; You get to select personal options to either use or discard, directly out of hand. Kinda like a buffet.