My best guess would be one of your other Severums. Do you have a mixed severum species within the four? I had a slightly larger gold severum that would pick on and fin nip my slightly smaller red-hooded severum.
I would all but rule out you 15" arrowana. You would have at best been burying this fish instead of treating nipped fins in a hospital tank. I have never kept arrows, so this is coming from hearsay and not personal experience!
Next, I would all but rule out your 7" bichir. This is coming from some limited experience (< 2 years) in keeping various bichirs. The bichir bites that I have seen are generally directed at the torso of the victim. I do not believe that this is the case. IMO you would see something that looked more like a round b.b. hole in the side of your fish rather than reipped fins. Given the death roll that the Bichirs do when they get a firm bite, I would imagine it would have effortlessly removed the entire fin. They would be missing, not ripped.
I see some possibility of any of the others though. The dolphin cichlid is african and often CA/SA cichlid and African cichlids do not mix well. Again going strictly off hearsay with this species, it is my understanding that african cichlids are considerably faster swimmers that CA/SA cichlids. In general it is considered that the african cichlids are better all around fighting fish (given equal sizes) than their CA/SA counterparts. IMO the african cichlids that I have kept are far more agressive than any Severum that I have kept. The dolphin being smaller than the severum could have very well been the savior of your far less agressive severum.
Guramis I would think too slow to do this kind of damage to a cichlid, but who knows, with enough persistance possibly it could have caught the severum.
Never kept a RTC, but the RTC's are just simply legendary when it comes to picking on tankmates. No way to rule out this fella!
