Angelfish
Angelfish are cichlids, and that is the thing to hold onto. They are sold as peaceful community fish at the size of a coin, and they grow into territorial animals with opinions.
Tall tank, not a long one
A mature angelfish is six inches long and can approach eight inches tall with fins extended. Height is the constraint, not length — a standard shallow tank cramps them and their fins. Aim for a tank at least 18 inches tall, and 20 gallons minimum for a single fish, more for a group.
What happens when they pair off
Keep a group of juveniles together and eventually two will pair. From that point the pair claims territory and the tank changes: they will drive other angels away from their corner, and in a tank that is too small, drive them into the glass. This is normal cichlid behavior, not a sick fish. Either give them space, or plan to move the others. Six juveniles raised together in a large tank is the usual approach, accepting that you may end up rehoming some.
Tank mates to think twice about
Those long trailing ventral fins are an irresistible target. Serpae tetras, tiger barbs and other known nippers will shred them. In the other direction, an adult angelfish will eat neon tetras — not out of aggression, simply because they fit in its mouth. Cardinal tetras and larger rasboras are the safer version of that look. Corydoras, larger tetras and most peaceful bottom dwellers are fine.