This booklet covers a wide range of topics that will help maintain quality seawater in marine aquariums. It begins with a discussion on the freshwater used for preparing seawater salt mixes and/or evaporation make-up. Processing equipment is explained, as is the use of distilled and well water. Synthetic salt mixes and natural seawater usage is also discussed, as is salinity, specific gravity, conductivity, osmotic balance and applicable testing. Elements and compounds of importance are explained, e.g., Ammonium/ammonia, nitrite, nitrate, pH, phosphorus/phosphate, oxygen, carbon dioxide, alkalinity, calcium, magnesium, strontium, silica, iron, fluoride, bromide, iodine (including iodide, iodate and Lugols), boron, molybdenum, copper, and trace elements. The booklet closes with one of the most comprehensive tables of seawater elements ever published. An easy to read booklet that takes the mystery out of maintaining quality seawater in your aquarium.