Dwarf rainbow fish. Could they be breeding already

[skargo2009]

So I bought 8 dwarf rainbow fish yesterday (5 males and 3 females). It's all they had and I didn't want to just buy 6 and leave 2. Anyway they have settled in well. I'm not sure but they appear to be showing mating behaviour.
The bigger males keep chasing the smaller males away from the females, and then they appear to be displaying to the females. When they are displaying the males look like they have a thick yellow line running down from the end of their dorsal fin to around the tip of their head

 

[Cecibella123]

I've had occasions where fish bred soon after putting them in quarantine. I'm not saying that yours are breeding, as I have no experience with dwarf rainbows, but I certainly wouldn't discount the idea. Keep an eye out for eggs so you can take appropriate precautions if you want to rear the fry.