Gash or cut like line

[Ncreefer]

Have a 75 gal saltwater tank- a cardinal just swam across tank and ive noticed on his left side of body he has a white colored gash or cut like looking line running downward from top to bottom. Any clue of what this is or why

 

[Hinds04]

Any tangs in the tank?

 

[Bonessurf]

Why a tang?

 

[Hinds04]

Tangs have spines on there tails that they use to slice other fish basically... Usually other tangs but I had a royal gramma that got sliced from the top of his head all the way down his side about 1/2-3/4"