I may be off base here but I don't think a 1.5 g sump is gonna be worth the effort. Usually people out heaters and skimmers and macro algae in theirs and I just don't see there being room. Plus you would have to drill your tank as even a nano overflow will take up alot of space.
If it were me, I'd just do weekly water changes, use an aquaclear 30 for filtration and add a heater. And of course the rock and sand.
You just really have to be up on cleaning the tank and monitoring the water parameters cause sump or no sump with that much water volume you can run into trouble really easy and quick.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.