What is the average feeding amount of a fish?

I agree - I believe it's honestly really about trying to feed the fish/fishes in your aquarium, with the correct diet, and to their feeding needs. Fish that don't have any stomachs (as you noted, like Goldfish) will need to be fed far more frequently compared to fish that do. Temperature and other conditions can also play a factor in feeding since many fish during colder temperatures, may not actually need any food (or very limited) as their metabolism slows down.

Quote from Muneeb Naseer

When it comes to the actual amount of food you should feed, the first thing to consider is the type of fish you own. Different fish require different amounts of food, and so you should try to stick to the guidelines given by the fish breeders themselves. If you own a goldfish, for instance, you should feed it about five times a day, for about a month and a half, and then three times a day for the next two months. Then, you should feed your goldfish about twice a day for the next six months and then once every three days for the next six months.

I really had no idea about goldfish and the high amount of feedings! No wonder my carnival wins never lasted!

I agree that size and type of fish would correlate to how much food is necessary. Overfeeding will cause a lot of crud to build up in the tank and underfeeding will starve them.

This is something I feel many people are not aware of when they get a fish.

Fishes such as goldfish are often either bought for kids as a pet or are won at fairs and carnivals and a lot of the time, they don't always last too long because many are either over fed or underfed.

I believe that is one of the common mistakes made by many when it comes to fish.