im glad we are talking about this topic.
your sump is going to want to be media free. saltwater filtration is different then freshwater. in fresh the idea is to trap debris/food/poop ( in a filter, floss, bioballs... ) until it can break down into nitrates.. in saltwater you want good flow (powerheads) and a good skimmer. the constant water movement with little to no dead spots keeps what normally would be breaking down in suspension in the water until the skimmer can remove it. this means NO filter floss, bio balls, canister filters, hang on filters, even filter socks that dont get changed frequently cause this problem. http://www.tropicalfishkeeping.com/s...shwater-31955/
since your using the 20 gallon as a sump im going to assume your building the baffles? i personally would have a skimmer section, a refugium section ( with chaeto algae and a clip on shop light with a daylight flouro bulb ) and a return pump section. the return pump area should have a bubble trap before it, and this is also where you will notice evaporation. http://www.tropicalfishkeeping.com/s...g-sumps-15930/
i personally would do a 4 - 5 inch even sand bed in the tank. this can be tricky too. sand to small in grain size will compact but things to large ( like crushed coral ) allow debris to fall in between the nooks and crannies causing issues like mentioned before with freshwater filtration. i wouldnt stress to hard over your sand being to small though, usually they're all good. ( sand directly off a beach would do this, plus bring polution and toxins into your tank, and more importantly may be illegal to do so )
i think your also going to have to go with more rock. im going to say atleast 80-90 lbs. which can vary depending how pourus the rock actually is. the more the better, but not so tightly packed ( esp not in a sump or refugium, it acts just like a pack of bioballs ) to allow good flow throughout. speaking of flow, i personally like hydor K powerheads. 2x #3 or #4 hydors on either side of the tank and you should be good. i like jager heaters too.
how will you be getting the water from the display to the sump? hang on overflow box? drilling? reef-ready tank?
it may be a good idea to look in your area for a local reefing club.