Look at how many fish are in this tank. There is fish of all shapes and sizes, a dozen species and hundreds of thousands of individuals all in this tank. Yet, they are all doing fine. If you tried this sort of thing with land vertebrates, they would become extremely stressed and many would die. What is the explanation for this? Are tetrapods more prone to stress than other vertebrates?
I believe for fish the most significant stressor is probably water quality rather than number of individuals, at least for open water species - fish that live on the sea bed are often territorial and then number of individuals becomes a key factor. If it is not possible for many reef fish such as damselfish to defend their territory extreme aggression and fatalities are a common result.
So why are pelagic fish so tolerant of other individuals but not land vertebrates or benthic fish? Maybe it's the fact that the pelagic zone has more space, but there must be something else.
One possible reason is the difference in food sources. A fish that eats plankton must go where the plankton goes and provided it is in the right place, its food is so abundant that it doesn't need to worry about competitors. But a fish that eats benthic algae and invertebrates may benefit from defending a territory from others. Something similar applies to breeding behaviour, pelagic species often spawn in massed shoals, but benthic species may form pairs and practise some form of brood care. I'm sure there are many other factors too.
It's an obvious explanation but I just thought I'd mention it since it doesn't seem to have been stated here and obvious explanations are usually the best... fish can go up and down in a tank in a 3D space but terrestrial animals are in a 2D space.
It is because fish are simply overcrowded, they are not meant to bred and dying ones are replaced regularly. Zoo exhibits 100 years ago were similarly overcrowded: http://static3.akpool.de/images/cards/170/1708740.jpg http://www.thelearnedpig.org/wp-content/uploads/2017/02/Fig.06.jpg http://static4.akpool.de/images/cards/152/1525444.jpg I think aquariums in the next years will also go for better husbandry, low density, high longevity and regular breeding.
Some of the best tanks I've seen have been lightly stocked, anyway. I've been to see the Georgia tank and even really big, significant fish just get lost in it. It's really just about the whale sharks and manta rays, with 100,000 filler fish.
Many species defend a territory or home range, some as individuals others collectively (eg tigers and lions) - although obviously this may be linked with mating rights as well as feeding rights. On the other hand wildebeeste migrating across the Serengeti and Maasai Mara are more like pelagic fishes, because they do not need to compete for grass, they simply keep moving to follow the rains and usually find abundant food.
