Just as much as your fish can consume in 1-3 minutes. Keep in mind that their stomachs are about as big as their eyes.
For a healthier fish, feed him pure protein once a week such as brine shrimp, meal worms, and blood worms. I suggest getting frozen cubes instead of freeze-dried selections, but get live food if you can. After you feed them the protein, fast them for one or two days so that their systems can be cleared out.
Feed them once a day.
Good luck!
~Also know that Comets need up to 1,000 gallons when they reach full size.