Rainbows fighting or courting?

[squealor]

I just bought three Boesemani rainbow fish two days ago. We've tried sexing them to no avail. Two are smaller than the third, but other than that, the coloring is exactly the same. They are all young, around 1 1/2 inches long. As soon as they were in the tank, they seemed to get along fine and fit right in with all the other fish. They are eating well.
Tonight, we noticed that two of them (the two smaller ones) were chasing each around in a very tight circle, bumping into each other and nipping a bit. Each of them took a little bit of damage to the fins, but not much. This went on for quite some time. During this, the colors on the two fish became very vivid and dark... the front of the fish becoming a very deep blue/purple and the back becoming a bright yellow-orange. The third fish seemed to have nothing to do with any of this and just swam around like normal.
My question is, are they fighting or are they courting? I got a bit concerned while it was going on, but now all three are hanging out and getting along once again...

 

[TankGirl]

I'd wager that these are two males. When it comes to rainbows, most of the action you see will be between males, and unlike many other fish it is recommended that you keep more males than females, so they will not beat up the females for lack of males to "fight" with. This activity rarely results in serious damage.

I would encourage you to get a couple more of them, of whichever gender, as they will be much happier, though they do reach a decent size and have a larger impact on bioload than other rainbow species. I keep them and they are vibrant, active, stunning fish - excellent choice for your tank!