i recently set up a very similar cube tank. my tank is 24x24x20, built out of 3/8" acrylic. there is an external overflow, and i have about 3" of eurobrace around the top.
so far, my only problems with it is that i would redesign the sump to accomodate a stronger return pump, and i'd split the return plumbing to come up to the front 2 corners. i have 2 nanostreams (6025's) in the back two corners, and the flow is strong enough to kick up the sand in the front.....but not too bad when i put enough rock up front.
i was thinking about putting 2 nanostreams (6025's) also in the front, but i dont want them to be an eye sore in the front of the tank. so eventually once i get a new redesigned sump, i'll put in those return lines to the front corners (the nice thing about having eurobracing).
also, my back wall is actually 5/8", and the magnets on the nanostreams are BARELY holding them up.....so its gonna be challenging to find magnets strong enough to hold any sort of frag rack.
my only other problem has been trying to figure out how to clean the inside of the acrylic without scratching it.....sigh. any ideas?
i'll put up a pic in a little while.