Artificial Seawater and Commercial Freshwater
Fortunately for aquarists, synthetic sea salt mixes are now readily available and provide a very stable and healthy medium in which to raise marine fish and invertebrates. Most come complete with trace elements, which should be replaced periodically per the manufacturer's suggestion.
If you have tap water of poor quality and a large number of aquariums, you might consider the use of a reverse osmosis filter. Salts, minerals, and metals are removed from the water through the use of a fine membrane. These units are, however, quite expensive.
The tap water used to mix the seawater for marine aquariums and for freshwater aquariums must be tested for nitrate and phosphate levels. High levels of either can cause rapid algae blooms. Tap water also contains chemicals such as chloramine and chlorine, which are added to help make the water safe for drinking, but are toxic to many aquatic creatures. Fortunately, commercial preparations that instantly remove both chemicals are readily available. Copper is also toxic to many creatures, particularly invertebrates (copper is often used in medications designed to kill various invertebrate parasites of fish). Water that travels through copper pipes, today usually found only in older buildings, is generally unsuitable for use in aquariums. One alternative if you have tap water of poor quality is to use commercially available spring water. This is generally only an option when you keep a small aquarium. Do not use distilled water, as it is completely free of chemicals and, through the process of osmosis, will cause essential salts and trace elements to leach from the bodies of your pets.