He has over flow boxes in each corner where the lip on the 2 sides is about 1/4 inch lower than the maximum height of the water by the looks of things. He then has pipe stands one is the emergency over flow which is at the same height as the lip for the over flow boxes the over pipe stand shorter one is the main drain pipe. In case of a power outage only the first 1/4 inch of water from the DT can flow down to the sump. You have to make sure that the sump is large enough to hold the extra water without over flowing. When the power comes back on the water will then return to the DT and flow over in to the over flow boxes and in to the sump. You use stand pipes to make the over flow quieter almost to the point you don't hear it. There is different was of building overflows you can have an external one that you drill the top rear of the tank. Or as I will be doing a back wall over flow but being it can eat up real estate I am making mine only the top 6 to 8 inches of the tank which will create a lip over the bake of the tank on the inside this will inhibit algae growing on the back wall a bit. I can put mushrooms along the back wall they will grow ok there. It all depends on what you want your tank to look like and how much effort you want to put into it. What I am going to do will be harder but more efficient in the long run.

I seen a web site that gives good advise on sumps and how to make one step by step.
I will try and finf it and post link her for you.

I found a couple of web sites.

http://saltaquarium.about.com/od/sum...mpoverflow.htm
http://avdil.gtri.gatech.edu/RCM/RCM...umpDesign.html
http://saltaquarium.about.com/od/diy...Sump_Plans.htm
Bill