question on dog fish

[mistypilosa]

my dog fish turns black why??

 

[fbm]

I don't know anything about dog fish.
However some of my fish loose color at night (normal) or when they get stressed. My fish don't like to be startled and when they do they loose color.
I believe this is normal for most fish.

 

[f14peter]

Along with what FBM mentioned, some fish also change color respective to the color of their surroundings.
My blennie is light tan when he's on the sand, a greyish-brown when he's working the rock, and very dark greyish-brown when he'd bopping along the black sides of the tank.