Boesemani rainbowfish: only one side of head has turned black

[MrsE88]

Very possible, could also possibly be a mutation as well. It could even be a accidental hybrid to. Personally I never own that specific specie before, I've only ever had Australians and an albino millennium red.

The OP should test their water parameters and post the results to rule out any underlining causes. Personally I'm still leaning towards the maturation process but I could easily be wrong too.

I didn’t think of that. Could be a mutation if it’s otherwise healthy.
My male has never gotten as dark as the picture from google up above. And he never stays dark, just what he’s showing off for my female rainbow

No damage. If you look closely, it looks like someone has painted the black colour..

Is it always like that? Or does it come and go?

 

[Magnus_Bane]

I didn’t think of that. Could be a mutation if it’s otherwise healthy.
My male has never gotten as dark as the picture from google up above. And he never stays dark, just what he’s showing off for my female rainbow



Is it always like that? Or does it come and go?

I noticed there's atleast a few variations in color in this specie. Some end up like the ones I showed while some end up more orange then yellow and get a dark blue head instead. I've even seen a few where they are more red then orange or yellow. I imagine it has to do with line breeding more then anything else. I'll post some more pics of the other 2 color types I see allot.

 

[Aanand]

I didn’t think of that. Could be a mutation if it’s otherwise healthy.
My male has never gotten as dark as the picture from google up above. And he never stays dark, just what he’s showing off for my female rainbow



Is it always like that? Or does it come and go?

This is the first time this has happened. Its been black for 2 days. 3rd day, i.e., today, the area near the eyes have cleared up a little bit.

 

[MrsE88]

This is the first time this has happened. Its been black for 2 days. 3rd day, i.e., today, the area near the eyes have cleared up a little bit.

I would just keep an eye on it then and keep up with water changes to keep nitrates low.