An article in the New York Times relates new findings on aquarium fish behavior. Turns out they are much more aggressive than their counterparts in the wild:
One thing the article does not address is differences between species in their responses to aquarium overcrowding. Solitary species must surely get more aggressive than social fish who thrive in schools.
My brothers and I kept freshwater aquarium fish for many years, and we did lots of research on what kinds of fish to get, making sure we get species that like the same water and temperature conditions, and were social and non-aggressive. Many of the fish we used to keep were smalls ones that were happiest in schools, like South American tetra species, or Asian species like the zebrafish and the cherry barb. The optimum size of the school for a given fish tank volume must differ between species.
The fish used in the study, the Midas cichlid, is by nature territorial and aggressive, so the effects of living with other fish in a small fish tank on aggression must be more acute in this species that in others. Still, the study is important in helping us be aware of the silent stress that fish may endure in aquarium tanks. Maybe it will point us to what species are suitable to aquarium life and which are not, what the optimum tank sizes for different fish species are, and in what numbers of fish to keep them in a given volume of tank.