Pale Zebra Danio?

[FurFinFeathers]

I've recently been noticing that out of my five danios, one of them has much duller stripes than the others. He acts normally, but just has duller stripes than the others. All of the other fish in the tank are perfectly healthy, and he acts perfectly healthy. Is it something to be worried about? Could it just be genetics or something? Thanks.

 

[McCool]

Could be stress. What is the ammonia, nitrite and nitrate? What size is the tank?
I had one or two danios in my school that were duller then the rest, so it is possible that's just how he is.