A lot of your finer particles that need removed will float in saltwater, with the proper flow will go into the overflow. Also, said system will have a cleanup crew (crabs, pods, worms) to eat the particles that don't make it to the surface. That's what they are for. If the system contains coral, it should also have a heavy, random flow. That would assist in suspending particles to get to the overflow where they can be filtered out. Overflows also assist in adding oxygen to the water column by the flow itself.
You don't mention if this system has a substrate. That may affect where you draw water from. Especially depending on the size of the substrate particles.
If you have something drawing water from the bottom of the water column only, the surface of the water is not getting cleaned. Also, I would think there should be some sort of filter in front of the intake to prevent fish and what-not from going down the drain, which would create something else to maintain.
Many tanks built for saltwater have a layer in front of the overflow itself with additional intakes at the bottom to help draw from the entire water column.
Detoxifying the water can be done many ways. Wet/dry, sump with a skimmer and mechanical filters, bio-balls, live rock, GFO, carbon, etc. It all depends on what's in the tank. If the tank has SPS, it's critical PO4 is kept at a lower level than what would be required with fish only. If the tank has a lot of large fish, such as tangs, poop removal is most critical since they are essentially nitrate factories (unless you like algae).
This article discusses surface layers and agitation and why it improves the health of your aquarium: http://www.advancedaquarist.com/2009/6/beginner