Mystery fish!

[Colin_T]

The male G. incisus in the pictures is simply a big old male. They get that shape when more than a few years of age. It's probably about 5 or 6 yrs old. They can live for 9-10 yrs.

 

[Sanityassassin]

I agree with @Colin_T. I've kept irian red rainbows from juvenile up to adulthood. Young fish have no hump whatsoever but as they mature the hump, as in the picture, is very prominent. That particular picture looks like a gnarly old male of a few years of age.

A lot of the rainbowfish species have this humped trait. I've had Boosemani, Turquoise and Irian reds and the old males were all the same.