I've had up to 6 adult rotkeils, few geos, few guianacara, etc. in a 6 ft 135 gal tank with no problems. Had two pairs with the rotkeils eventually and when time came to spawn for whichever pair, they'd just pick one end of the tank, so still not a problem in a 6 ft. tank. Occasional minor chase, sometimes a bit of fussing, etc. but no big deal, nothing you wouldn't get with angels, discus, geos, etc. It's certainly possible to get a mean severum but ime tank size makes a difference with them.