Help with Harlequin Rasbora colour change.

[Bobadubub]

One of my Rasboras has developed a grey/black coloration on its head and all the way along its top like a stripe. It's hard to describe, I have a picture but I don't think I can link to it as a new account (?)

I've had their school for 10 months and this is the only one with these markings, and it's only just developed in the last few days.

Water parameters are fine. 0 ammonia, 0 nitrite, ~10 nitrate, ~7 ph
Tank is 200L (~43 gallons) and understocked.
I do 30% water changes weekly.

Behaviours in all fish (including this one) seem normal; the fish are eating fine, swimming fine etc.

Their have been two changes in the tank lately that I can think of:

1) I bought a couple of new types of food for variety, some dried daphnia and a little bottle of brine shrimp suspended in liquid. I have also fed the tank blanched peas on a few occasions which is something I've not done in the past.

2) I opened the venturi valve on the filter for the first time over the week between water changes last week, but it is closed again now.

-------------
I'm not sure if this is something disease related or just a normal pigmentation change?

If it is normal, why did it only just happen now after all this time?
If it is not normal, can anyone help diagnose it and/or suggest treatment?

Thank you for your time!

 

[Barliman]

If he's not showing any changes in behavior (shy, refusing to eat), I wouldn't worry. He could just be a genetic sport with different coloring.

 

[Bobadubub]

A genetic marking that just suddenly appeared after 10 months?
Is that a thing?

but yeah, I'm not going to act on it until I have a more conclusive idea that there's a problem.

I know with some species their colours can sort of 'come in' after a settling period (no idea if that's true with Harlequin Rasboras) but this fish has been happily settled for months and months, no obvious problems or signs of stress.

 

[Barliman]

A genetic marking that just suddenly appeared after 10 months?
Is that a thing?

Goldfish and bettas can change a few times in their lives. I haven't seen it with rasboras, but I wouldn't be surprised.