We can not assume that fish in the wild find their food at at the same time every day and it is always plentiful. They just tend to gorge themselves whenever food is available. This is their survival instinct
We all know no matter how well fed our fish our are, whenever we feed them they act like they are starved.
This is pretty well said
Fish and other animals are opportunistic feeders meaning they eat whenever food is available, and they eat as much as possible when it is available because they don't know when their next meal will be. People were the same before modern technology and modern farming practices provided some of us with a stable regular food source. However, there are plenty of people around the world that still struggle to find sufficient food on a daily basis.
In the wild most animals, birds, fish, insects and reptiles have periods with lots of food and periods with little or no food. I used to collect fish and in the dry season there would be lots of small flying insects for them to eat (great for surface dwelling species, not so good for bottom dwellers) but less water to swim in. In the wet season there would be lots of water but fewer insects. In the wet season food was more spread out and the fish's diets consisted of a lot more variety.
Humans and all animals do best when fed a varied diet that is nutritious and balanced but not consumed excessively. You have to remember that 100 years ago people didn't even have a steady food source or large amounts of food. Refrigerators didn't become readily available until the 1960s and prepackaged foods weren't common until the 1970s & 80s. Before that you bought meat from a butcher and veges from the fruit n veg shop and cooked the food that day or the following day. If you didn't have a local shop to buy these items, you hunted or grew your own. You might not eat a decent meal every day and fish are exactly the same.
Fish that are overfed will gain weight and fat fish are not as healthy and do not breed as readily as slimmer healthier fish. However, fish do need to be fed well to produce good quality gametes (eggs & sperm). Young fish should be fed well until they reach sexual maturity. Fish being used for breeding purposes should be fed well for several weeks before breeding. But adult fish just hanging out in a tank should be fed a moderate amount so they don't gain weight but aren't starving.
Fasting healthy adult fish for 1 or 2 days a week is not going to harm them and may actually help them maintain a healthier weight, as well as have a healthier body in relation to how their body breaks down food and uses the nutrients it gets.
My fish were fed well but would still beg for food any time they saw my bedroom door open or saw me entering the room. I could feed them all day every day and they would still act like they were starving. My dogs were the same, especially the bigger dog. They were fed well but always wanted more. There was only a couple of times I remember them refusing food. One time we had a power failure that lasted 5 days and I had bought a heap of meat a few days before. I had to use it up so it didn't go off. I cooked heaps and fed it to the dogs and family. After a few days of being offered all you can eat bbq meat, the dogs said enough is enough and actually refused food. They just lay there with huge bellies saying I can't eat any more