Macropodus (Paradise fish) are supposed to be aggressive and territorial, unlikely to tolerate others of their species. Siblings are sometimes described as being more tolerant. Having started off with a pair and gradually added several of their offspring into the tank, I seemed to have the perfect Macropodus society. Aggression was minimal, and I saw no evidence of the sort of behaviour I had read about. I gave away the first batch of progeny, and had the same experience when I added some more juveniles from the breeding tank.
My second purge was more extensive, and I ended up giving away all the large Macropodus. So when I added a new batch from the breeding tank, the effect was very different. They have remained much more aggressive with one-another, displaying at (and chasing) their siblings. No clear dominance hierarchy seems to have emerged either. Since it makes for far prettier fish, and since I haven't seen any evidence of fin damage or wounding, I am not too worried - just interested in how social dynamics seem to emerge.