I introduced O #2 (2-3 inch) when O #1 was 6-7 inches & they became fast friends, lived together for years, till I had someone babysit them for 2 weeks. but thats another sad story
what was the size of the tank? maybe it was too small and had nowhere to hide. conspecific aggression is an issue with most cichlids at one time or another
yogurt_21;3288165; said:see this is exactly why they say keep one oscar or keep 4 or more. as 2 of the same sex will kill each other opposite sex you may be alright as long as size isn't too different. my 12" red is male and my 10" red albino is female so they get along quite well, though the mating dance can get quite rough.
and even a breeding pair will kill each other in too small a tank.
this is true of any predators really.