When I first got ornatum about 2 years ago, they were approximately 1" and
started growing them out in a 55 gal, just the 6 without any other fish.

I found them to be very aggressive with each other at that tiny size, as it appeared they were on the road to killing each other off.
I added 6 slightly larger Guianacara sp, and a few lemon tetras, overnight this was enough of a distraction to halt all intra-species aggression.
Within a year they were too large for the 55 and were moved to the 150.

I probably should have reduced the population of Guianacara, and not had any more than 10 fish in the 150, and it would have been much better if I had had even fewer.