Saltwater fish need very precise water conditions that approximate the composition of natural seawater that is found in the coral reefs and oceans.
Seawater isn't simply salt and water. It contains many other elements, ions, and minerals. These are all necessary for the proper biological functioning of the fish and invertebrates you plan to keep in your aquarium.
pH and Calcium Seawater is somewhat alkaline with a pH between 8.0-8.3. You want to try to keep the pH in your tank as stable as possible. Thankfully, synthetic saltwater mixes contain buffering agents (bicarbonates) to help prevent wide pH swings.
Factors that may cause your pH to drop include poor aquarium husbandry, such as overfeeding, overstocking, and failing to do routine partial water changes. Another factor that may cause your water pH to drop is low water calcium concentrations because calcium ions combine with other ions in your aquarium and act as buffering agents that protect against pH fluctuations.
Low calcium levels can happen if you keep invertebrates in your tank, such as coral, that construct their skeleton from calcium carbonate. Fish also use some calcium, but to a lesser extent than invertebrates do. In the closed environment of the aquarium calcium levels will eventually fall. You can monitor calcium levels by using a test kit. You can also prevent calcium depletion in your saltwater aquarium, and the pH fluctuations associated with low calcium levels, by using a calcium reactor.
You should test your water calcium and other water parameters frequently. Calcium levels should be about 400 mg per liter. If your water calcium drops too low you can increase it with a liquid calcium supplement.
Temperature The temperature in a marine aquarium should be between 73-79 °F (23-26 °C). Keeping it at the lower end of this range may increase the longevity of the fish because their metabolism will be slower.
Other Elements and Ions in Seawater Seawater contains many other ions and elements necessary for the survival of fish and invertebrates. Some of these include strontium, iodine, chloride, sodium, magnesium, potassium, zinc, copper, chromium, and others. The best thing that you can do is to get a good quality saltwater mix that contains all of the necessary trace elements and ions. There are several good sea salt mixes, but I use Instant Ocean sea salt. I think it is one of the best and it contains all of the necessary elements and ions to approximate natural seawater. But any saltwater mix from a reputable company should suffice.
In the closed environment of the aquarium, ions and trace elements get depleted. The water in the aquarium also becomes more acidic over time due to waste products from the fish, other tank inhabitants, and decaying food. This is why partial water changes are necessary to keep your fish and invertebrates healthy.
Also, don't forget to test your water parameters on a regular basis. They sell test kits for all kinds of water parameters including test kits for ammonia, nitrite, nitrate, pH, calcium, copper, phosphate, and other elements and ions. You will also need a hydrometer to measure the specific gravity of the water.