I get what you are saying here trust me i do but if it were just about best interest of the fish they wouldnt be in a glass box in our house, for instance in the wild neons live in shoals of 100s yet in my 20 gallon i only have 10, sure 10 is better than 6 and way better than 3 but whats best for the fish is for it to be in a river in the amazon in a shoal of 100 neons. Part of fish keeping is making concessions on whats best for us and whats best for the fish and we usually win out because whats best for the fish isnt being kept in a glass box in our house.
I'm sure you knew I would pick up on this.
I don't subscribe to this at all. There is enough scientific evidence that aquarium fish can be as healthy and well balanced in an aquarium as they are in the wild. The issue is knowing what the individual species "expects" and needs, and ensuring that is provided. Providing for that is meeting what is best for the fish. Obviously we still cannot know how the fish "thinks" about all this, and we never will unless fish learn to speak our language. It is easier to read signs in mammals than it is in fish, but that does not discount the fact that we can know the expected results of providing or not providing what the fish deems critical to its well-being.
Lifespan is one measure of a fish's well-being, perhaps the most important because it depends upon the fish's physiological functions and as these are internal it is impossible for any but a qualified biologist to discern these upon necropsy. A lifespan equalling or surpassing the norm for a species should mean the fish is in good health. And good health means little stress. Stress causes 95% of all disease in aquarium fish. Understanding the species' natural behaviours and requirements is key. The closer we approximate the needs of the fish, the less stressed the fish will be and that means better health.