The most obvious difference between a freshwater and a marine tank is the addition of salt to the water. A freshwater tank needs only clean, chlorine-free water. Saltwater tanks require the addition of marine salt, which can be purchased at aquarium supply stores.
Unlike table salt (sodium chloride), marine salt contains a wide variety of soluble minerals that are essential for marine life. There are a variety of marine salts available, and your choice may depend on the type of species you would like to keep. A hydrometer is used to measure the amount of salt in the water as specific gravity. Once the aquarium contains the correct amount of salt, it can be topped up with fresh water unless you perform a water change; although the water itself will evaporate, the salt will not.
Most freshwater tanks require some form of filter – often a combination of air or water pumps and filtration substrate such as sponges, carbon, or gravel. In addition to this, marine tanks need a protein skimmer, also known as a foam fractionator. These devices use bubbles to froth up and remove contaminants from the water that are too small to be removed by conventional filters. They are needed to maintain the appropriate chemistry in the tank – marine species are more sensitive to nitrogenous waste than freshwater species.
The ocean is much larger and more stable than the average lake or river, and as a result, marine species are far more sensitive than freshwater species to changes in their environment. Optimal temperature should be determined by species and monitored closely. Measures of water chemistry, such as pH, phosphate, nitrate, calcium, and others, should be performed frequently in a new tank, and occasionally in an established tank. Invertebrates such as snails, anemone, and coral are even less able to adapt to changes in water quality than fish species.