The true specific gravity of seawater in the 28 to 35 ppt range is 1.0206 to 1.0260, and most marine aquarium systems fall into this range. It is good to keep the salinity of a marine aquarium a bit lower than ''normal'' seawater. Never add saltwater to a marine aquarium to make up for water lost to evaporation. Water is pumped through a pad or wad of floss or other filter media, where the particles are trapped. The filtering media is then cleaned or discarded after a week or two to prevent the accumulated particles from clogging the media and reducing the flow of water through the filter.
Biological filtration is the cornerstone of a marine aquarium.
If an aquarist does not understand and properly utilize the process of biological filtration, a marine aquarium will forever be a thing of mystery controlled by supernatural forces. Marine animals and organic decay produce waste nitrogen in the form of ammonia. Ammonia is toxic to marine animals, and many fish and invertebrates become sick and die if ammonia is present in the tank water. Certain bacteria convert ammonia to nitrite. Nitrite is also toxic.