Hyphessobrycon megalopterus male
Hyphessobrycon megalopterus female
I have 4 Black Phantoms, 1 male and 3 females. The females are easy to distinguish from the males as they have some red fins, while the male has all black fins. My male is a wimp and seriously henpecked. He gets bullied by the females, especially at feeding time.
They're great fish, although they shouldn't be kept with slow moving fish, as they will nip fins, and should be kept with fish their own size. Males are normally more strongly coloured than females. Sometimes, they will throw up their fins and circle each other, this is to establish territory. Mostly, they are an extremely peaceful fish, and I would definitely recommend them.
I've just bought six of them, four males and two females I believe, for my 60 L tank (alongside Neon Tetras, a young Angelfish, and a Red-Tail Shark) and although they don't seem to bother the other species, they fight amongst each other constantly. The males fight each other, nipping each other's fins, and occasionally they gang up on the females and chase them around (though there seems to be less fin nipping). Some of the males' fins are already in tatters and they've only been in there two days! I hope they settle down and establish some sort of pecking order as they're beautiful little fish.
I obtained 6 Black Phantom Neons - all male I think - the other day and introduced them to my tank of 20 mixed tetras. The Phantoms seemed very territorial and aggressive in my large planted and landscaped tank and were very unforgiving to all inhabitants. Anyhow, they constantly harrassed each other and the other Tetras, and nipped many a fin. Not the passive fish I had been led to believe. They had to go!
We had three of them. Two females and one male. When we introduced 3 panda cory cats, the phantoms went crazy, swimming right over the corys and occasionally charging into them. They were peaceful about 50% of the time. The rest of the time was spent bullying and chasing smaller or weaker fish. The male and the larger female turned on the small female, so we decided to take them back to the pet store. They were nice enough to let us trade them in for 3 much more peaceful platys.
I have two female phantom tetras. One of them seems to be territorial. I have a stump with a cave in the center of it. She spends most of her time there. If any other fish (including the other phantom) comes near her home she chases them off, although the other phantom schools with my lemon tetras. Weird.