Seachem makes some great filter media. Most can be run just by putting the filter bag into a section of your system that has water passing over it. For more information on a specific filter media click here.
Purigen: Is a synthetic filter resin that removes organic waste (ammonia, nitrite, nitrate). Never use Purigen in a tank that has not been cycled. One 100 mL bag treats 100 gallons. For use in fresh and salt water.
PhosGuard: Rapidly removes phosphate and silicate from your tank. If you are having an algae issue then this will help. Phosguard is spherically shaped to promote water flow around each bead. One 100 mL bag treats 60 gallons. Do not use in tanks where a phosphate based buffer is being used. Safe for use in fresh and salt water tanks.
PhosBond: Rapidly removes phosphate and silicate from your fresh or salt water tank. Phosbond is more of the traditional granular phosphate remover that you are probably used to. Each 100 mL bag treats 80 gallons.
SeaGel: Combines PhosGuard and activated carbon to remove phosphates, silicates and organics from your water. It is ideal for reef and planted tanks. 250 mL treats 60 gallons.
Matrix: Is a high capacity bio filter that removes ammonia, nitrite, and nitrate. While it will never bring nitrate to zero in tanks stocked with fish, as nitrate production is so high in these tanks, it will help to maintain more manageable levels of nitrate. If you want to remove nitrate from your system consider using Purigen in addition to matrix.
Rivers to Reef says: It is very common to run Purigen and Phosguard on reef and planted tanks to help keep algae down. Sometimes this can make the water "too clean" so if your coral and plants start not doing as well, consider pulling it out of your system. Test for nitrate and phosphate periodically, and when it gets to a level that makes you uncomfortable, put it back into your filter.