Yellow Tang lateral line

[Stackbundles]

So my Yellow tang is eating great and acting normal. Today when I got home from work, my tank lights were off and I notice a blackish lateral line, identical on both side of his body. I turned on the tank lights and within five minutes he was bright yellow again. Can someone explain this to me.

 

[KathyC]

What you noticed is completely normal in Yellow Tangs - Purple Tangs too. It is actually above their lateral line and not quite as long.
It appears at night or when they are resting, it's more like a rectangle that is a slightly darker shade than their body.

Nothing to be concerned with

 

[Stackbundles]

ok Thanks Kathy for the help.