Many fish that school naturally will not school within the confines of a tank. Yellow tangs are well known for schooling in the ocean, but fighting to the death in a tank, even a large one. Always use caution--in a large enough tank, adult chromis will school. In a smaller setup, the same number won't work. I've seen it work in tanks above 200 gallons, but never with adult fish in a tank under 100 gallons. The behavior of juveniles does not always carry through as adults.