In a freshwater aquarium, the filter acts in two ways. Firstly, it acts as a mechanical means of filtration by forcing water through media (sponge, filter wool, bio balls and many other types of media) to catch detritus, to be removed at each water change. Secondly, it acts as biological filtration, to provide an environment where good bacteria can live. These bacteria help to break down the bioload in the aquarium (as discussed above).
In a saltwater aquarium, it’s important to know that given the chance for detritus to build up, the result will be a rapid rise in nitrates. Therefore, if an external filter is not used and maintained correctly it can become a detritus trap and a major contributor to high nitrate levels. For this reason mechanical filtration media is very rarely used in saltwater systems.
Those who do use external filters are more inclined to use live rock rubble, for additional biological filtration, and products designed to remove nitrates and phosphates, as well as activated carbon to polish the water. Some keepers do use filter wool in their external filter to polish the water, but the wool is changed out every 2 to 4 days depending on the system to ensure it does not become a ‘nitrate factory’.
So how are saltwater aquariums filtered in the absence of a filter?
The answer is live rock.
Live rock is essentially porous ocean rock capable of sustaining your tanks population of good bacteria. Water is continually filtered through the live rock, allowing the good bacteria to remove ammonia and nitrite from the system. Your sand will act in the same manner, once live (although you can buy live sand).
There are of course additional methods of filtration to remove phosphates, excessive nitrates and impurities from the water, but the basis of any saltwater tank filtration is live rock.