In the past year that you have had them, has there been a noticeable change in size in one of them, i.e. one becoming larger?
If not, then they may be the same sex, and, as redvipe stated, the change may be occuring now.
If that is indeed what is happening, the one that is becoming larger is changing to female, and she will assert her dominance over the male in the form of violence. Hopefully she will accept him as a mate, and wont kill him.
Tank bred, for whatever reason, seem to be a little less violent than wild-captured. Also, ocellaris aren't the worst of them, which is also positive in this regard. If you were talking about a pair of wild-captured maroon and gold, then you would have to keep a very close watch on them.
If it goes on for longer than 3 days, and it looks dismal for the victim, then you will probably want to separate them.
