if you are going to do a DSB I recomend you read up on some of Bob Goemans sand mail comments in FAMA or maybe the archives at WWM - I don't think three inches is enough. The one in my 125 is something like this .75"pvc - .5" eggcrate - screen - 2" sand - screen - 2" sand. A little over 5 inches in total. I did this more then 12 years ago and I think they even recomend a little deeper these days. As far as the sump goes you have many options. I run mine BB with just pumps, skimmer, carbon canister, and heater in there it is by far the easiest to maintain. This also leaves room should you ever add a kalk reactor, top off system, or any of the other toys out there. Sumps do not make good true refugiums in that a pump is between the plankton and the tank, however they are often set up this way anyway with the light cycle in reverse to conteract the natural lowering of pH at night - so that definitly is a choice. I do agree with drty811 comment that sand in a sump could be a issue if you have a lot of flow through there. And since you have such a large return pump, I would go with an "empty" sump, and if you do ever add a refugium it could be a HOT type so the plankton flows into the tank - JMO ????? Lots a ways to build a better mousetrap eh?
