I currently have 3 in my 4ftx2ftx2ft, i noticed anoher thread were people said you can put 12 in a 6x2x2....i just want 1 more (a Rotkeil) on top of what i have.
Again, as long as your not bothered about breeding them, then your fine. I kept about 15 differant sevs in a 100 gallon for over a year (as the fish house was built) and never had a split fin (never had a spawn either mind)
In larger groups, they shoal exactly the same as discus and angels. Just keep an eye out for pairs forming and elevated aggression.
They have lots of large gorgeous rotkeils at Maidenhead Aquatics in St Albans at £49 each, apparently wild caught. They call them red headed severums but they are the same thing.