Sumps are incredibly useful devices. They increase the total system water volume, which makes your system's water chemistry more stable, without forcing you to purchase a larger display tank. They give you a place to perform water changes without disrupting your display environment, and give you a place to put things like heaters, carbon, and protein skimmers which look unsightly in a display.
Everything You Always Wanted to Know About Sumps Part 1 Part 2 Part3 by Greg Taylhttp://reefkeeping.com/issues/2003-http://reefkeeping.com/issues/2003-04/gt/index.htm,01/gt/index.htm,or frohttp://reefkeeping.com/issues/2003-07/gt/index.htmm ReefKeeping Online Magazine
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In a lightly stocked tank live rock and/or live sand "biological" filtration is all that is required (apart from the skimmer, that is).
In a more heavily populated "fish only" tank, a Wet/Dry trickle filter is probably the best biological filter, though it will lead to a higher nitrate level than live rock filtration.
Canister filters are good "mechanical" filters, but they tend to produce an excess of nitrates in the tank.
Under-gravel filters should be avoided where possible, as they also produce unacceptably high nitrate levels.
A sump – get the largest practical tank, plastic container, etc, which will fit below your tank (or anywhere else that’s convenient). A sump is not really essential, but it does increase the total water volume, and allows one to keep all the ugly equipment (such as heaters, chiller, skimmer, and just about anything else) out of the tank. It is also a great place to make a refugium, and it allows for a more efficient gas exchange due to the tumbling action of the water flowing down into it. A Refugium houses extra live rock and/or Macro Algea for exportation of nutrients.