How do you tell the difference between a male and a female?I think one has spots onits mouth area and one has lines,but I am not sure and have heard no documented proof to back up my theory.Do you know?About your question I recently kept what I thought was a pair in a 300 and they both fought all of the time and eventually killed each other.Undulates are fierce creatures and beuaties at that so be careful,I was warned by billsreef that that would be there fait and he was right.I think the only way long term to keep an Undulate Trigger is to keep it by its self.Goodluck Chris.