can anyone tell me this is?

[shark-boy]

hey guys i have gotten a new 4 foot tank and it has the attachment on the back. does anyone know what it is and how i would plug it off? (goes all of the way through... thank you

 

[RUBE31]

Pics please.

 

[shark-boy]

i hope this time it let me put the picture in haha

 

[coralbandit]

That's a bulkhead /overflow.
It can be capped from the outside if not used..