These fish like to eat other smaller fish like guppies and should be kept only with larger aggressive fish. I have 17 rainbowfish I keep in one 470 liter tank with 4 piranhas. They have formed schools and do not kill each other.
Very, very nice fish, one of my favorites also. After I bought 2 New Guinea (red) rainbowfish, I did not hesitate to buy 2 boesemanis. I bought these two 3 months ago and put them in a 320 L. Even though they are expensive in Mexico (US$16 each) they are great. They are fast swimmers and I strongly recommend them because of the amazing colors.
These are some really amazing looking fish. Mine tend to change colors depending on the amount of light that the tank is getting.. Dusky colored when the lights are low or off, radiant when the lights are on. Unfortunately they are a bit aggressive towards their tank mates. I started off with just one, and he attacked everyone. Taking a friends advice I bought a second, and now they have a good time chasing each other about the tank and knocking themselves senseless.
I love my 4 Boesemani's and am looking to add more to the school. They currently live with a friendly tiger barb (a rescue because he was being beat up by other barbs) clown loaches, otocinclus, chinese algae eater, mystery snails and used to live with some female guppies. I haven't had any problems with aggression towards other tank mates, though they are constantly displaying and showing to each other. They are just over a year and a half old, and I'm still not totally sure how to tell male from female. Maybe I have 4 males, as they all seem to participate in the showing off.
These pages have enough comments to give the reader a basic idea on the topic. Further comments are still very welcome (through the site's contact form) as long as they provide new and/or advanced information not yet discussed in the existing ones.