What are the chances....

[Delphinus]

I think you have to have a M/F pair for them to peacefully coexist long term. When I had this fish I bought two and while they didn't fight, per se, they didn't seem to get along very well, and only after one of them disappeared did the behaviour change to that of a more settled fish. I think they did actually fight and that I just didn't observe it, and eventually one of them just died. But yeah the behaviour of the fish when there was only one was remarkably different. Much more settled.

Supposedly, the pupil shape is different for males and females. Something about one of them is more oval shaped and one more almond shaped or something. Sorry I can't give better details but thought I'd mention this just so you can try some google searches if you're interested.