I think what you feed and what you are feeding are the determining factors. A whole fish or shrimp, swallowed whole will take longer to digest than pellets which take longer than flakes. A predator that has eaten a large meal gets lethargic, while a grazer never slows down. If you feed goldfish exclusively, the nutritional value is low from the start. My 20" knife and 10" Oscars get fed every 3rd day with whole shrimp or tialpia chunks injected very lightly with vitamins. As much as they will eat (stuffed). The oscars are fat, and the knife has grown 6" in a few months. My Africans get 3 of 8 different assorted pellets every other day, changing pellet type each feeding. My 5 gal nano with tetras, rainbows, shrimp, snails, and dwarf crayfish get fed flakes twice daily. Pellets are very nutritious, so daily feeding is too much. After years on the schedule, fish don't even beg on fasting days. My mbuna is very fat. The tetras and rainbows are midwater feeders exclusively, so the flakes drift around in the current for quite a while. Since they digest easily, not very nutritious, and the fish are grazers, twice a day the fish eat voraciously. Big tanks are bare bottom with an almost constant ro water drip, and water change weekly. Nano gets a 1gal water change 3 times a week. All tanks are over filtered. Every tank is different, every fish is different, every person is different.