Sump, missing a divider?

[Dmmuir]

Evening all,

I just got this tank, and it will be my first SW tank. The sump was filthy, so I spent a while tonight cleaning it up. It looks like on the left side there was another baffle, is that needed?

Since I'm new to this, how would you all set this up?

 

[Dmmuir]

And what is the little box thing on the 2nd picture for??? small hole in the top and on the front, nothing else opens.