GAC (Granulated Activated Carbon) is a great way to remove discoloration from the aquarium and absorb foreign matter from the water. This media is dust-free, but a quick rinse is always best prior to use. It can be used in a mesh bag, a reactor, or a canister filter.
Comes in Granulated or Pelletized form. Pelletized is better in a reactor because it won't sneak past the perforated plates.