For a reef tank you want a tank turnover rate of at least 3 times an hour and as much as 8. If you add a wave maker and a couple of powerheads in the DT you could be towards the lower end of turnover rate. But a rate of about 5-6 times an hour is in the butter zone. For your tank a 625 GPH to 750 GPH rate would be good and add the powerheads and some kind of wave generator, corals really love it. Like in previous replies above the pump should be rated at the head pressure your return is from a height standpoint. But since you have dual corner skimmer boxes, you will likely have a Y fitting, maybe a couple of bends, etc. all that will add to the head pressure. There are really complicated formulas, but an easy rule of thumb for a modest amount of fittings/bends is to add 20% to the straight head pressure. So if you have 5 feet of height/head you should use 6 feet for the flow rating. The rate of the drains at 1.5" depends on the height of water above the opening. Just a couple of inches below the water in the skimmer box can be 1000's of GPH. Even just 1/2" is about 400 GPH each. So that takes care of the size, as far as which pump, personally I like either the Sicce or the Lifeguard quiet one pro's... The DC pumps are nice, but either of those others are physically adjustable and once they're set you probably won't touch it again. They're all pretty high quality, but for me, durability and quiet running are most important. Heater wise I'd go with a single 500 watt titanium. And if you plan to put it in your sump and the sump has dividers, make sure it is in a section that will maintain water level no matter what. I've seen where people put it in the return section and low water caused serious issues. Hope this helps.
