glofish with red gills

[StuG]

So I added some glofish (zebra danio) to my tank the other day, and have noticed that at least a couple of them have bright red gills. It's the yellow ones I'm noticing it on, but maybe it just can't be seen as clearly on the pink ones?
My tank is cycled, and has been for quite a while, and my water parameters are good.
They are not showing any signs of distress (at least as of yet), and are eating well.
Naturally my first thought was ammonia, but it seems to be ok.
Any other thoughts on what could cause this?

 

[meegosh]

It could have been from wherever you bought them. What are your actual readings? What are you using to test your water params?

 

[StuG]

Ammonia: 0
Nitrite: 0
Nitrate: <>20

Tested with API liquid reagent.

 

[StuG]

NM.
It would seem from doing some more looking around that what I'm seeing is a natural marking.
Ah well, I guess it's better that I'm paranoid about this stuff than not giving a crap!