Which are the best filters for a marine tank?

Perhaps the most popular and widespread biological filter is the refugium. Water is drawn from the main tank to the refugium (usually a smaller container hidden behind or below the main aquarium), then is returned to the main tank. This smaller tank serves two purposes: it adds water volume (thus diluting any chemical problems), and it provides a place for biological filtration. Refugiums can also serve as a temporary shelter area for recovering injured fish, delicate animals that need to be slowly acclimated to the aquarium environment, and to temporarily separate fighting inhabitants. Refugiums often contain live rock and live sand, macroalgae, and scavenger microfauna such as micro brittle stars, tiny sea stars such as asterina, snails, and worms. Some people also use refugiums to raise tiny brine or mysis shrimp for delicate fish like seahorses and dragonets. A sump is a refugium that also contains the main aquarium's other equipment, to keep all hoses, filters, and heaters out of view. This is especially common for show tanks and reef tanks. An important thing to remember when considering method of filtration for a marine tank is planned stocking levels. A heavily stocked tank often will need strong biological filtration that can be provided by live rock and bioballs or other bio filtration media. A small tank only two or three fish can actually run very well with just basic undergravel filtration, (with bio balls in the uplift tubes)and no protein skimming at all.