yes that is the water draining into the sump from the tank above.
the larger sump area is to allow for daily evaporation. i lose about 2 gallons a day due to the halides, and i expect this to increase when i add a third lamp and another fan to compensate. also i tend to leave my girlfriend in charge of the tank when i go away, and i need plenty of room for error in case topping off water slips her mind that day. an auto-topoff system is in the works for the future, but it will need to take place outside of the stand anyway for space reasons. total calculated sump volume when filled will be about 10 to 12 gallons.
my first thought was to have the water level nice and high, and the bulkhead for the return pump low in the tank to allow room for microbubbles to rise to the surface, with the baffles coming from the fuge to the sump solving bubbles from the skimmer, but someone mentioned that running the skimmer outflow that high up will cause significant backpressure on the skimmer, possibly causing dramatic changes in its effeciency.
oh, the sump was conceived originally to be fabricated from a standard 29 gallon tank, but if i need to rework my skimmer discharge, i will most likely need to rethink that.
does anyone have any pics of their under stand setup? a 4 foot tank would work, mine is 4.5 feet long, so i have a bit more room. it is easy to work everything in if i am not trying to include a fuge, but when i add in the space for fuge and sump, an external pump, my skimmer and overflow container, co2 cylinder and calc reactor, and possibly a second pump for a closed loop system (currently considering dropping the main pump down a lot and going this route) space becomes a real issue.
still a month off from starting the actual fabrication, so radical changes and concepts are best worked out now.