-hang on filters and canister filters ( unless ran empty for flow or only with carbon these should stay on freshwater tanks. theres media reactors for carbon and powerheads for flow )
-sponge filters, bio wheels and bio balls ( avoid in saltwater all together )
all these are meant to trap debris which allow them to build up. on a freshwater tank this is the idea before doing a water change to lower nitrates.
-protien skimmer ( read reviews, some suck completely, some are worth more then gold )
-filter sock ( these can be just as problematic as freshwater type filters unless kept clean a few times a week )
-good flow with enough live rock
-some say a UV sterilizer but i personally do not suggest one, ( maybe the only exception is one on a QT )
the idea on a saltwater tank is to not trap debris, un-eaten food and detritus allowing it time to break down adn turn into excess nutrients. the idea is to have good flow to keep this stuff suspended in the water which will allow the skimmer time to pull it out. dead flow spots can become an issue as these allow debris to build up here and break down too.
this ofcourse is the very basic break down of filtration.