I am in the beginning stages of setting up my tank. I have a kent marine bio rocker however my tank is not drilled. So i bought a siphon box that hangs on the back so i am wondering will this leave to big of a whole that the octopus will be able to clime out of? i am getting a bimac if that makes a difference or not. The space would be about the same size of a Marine Land Emperor 400.

You really need to cover or in some way block any places your octopus could escape. You'll find lots of ideas on how to do this in the thead (under this Tank Setup and Maintenance topic) called "Do I need to block my overflow box".

Nancy