Equipment Needed
Most equipment used in freshwater can be used in a saltwater system, with a few exceptions. You should start by replacing your gravel with some sort of calcareous material. Examples include crushed coral, dolomite and aragonite. Using these types of substrate tend to help buffer the water and produce a more stable environment. Next, you need to check all your equipment for anything metal. Saltwater will rust anything except the highest grade stainless steel. There are stainless steels on the market which will rust when exposed to saltwater.
Needless to say, you need to replace or get rid of anything made of metal.
Filtration System
The filtration system used in your freshwater system will usually be adequate for a saltwater system. However, you can use this opportunity to upgrade or change filtration mechanisms. Also, which ever type of filtration system you are using, you should add some sort of extra water circulation to the tank. Saltwater has a lower dissolved oxygen content than freshwater, so you need to keep the water in the tank moving. Actually, it needs to do more than move. You need to disrupt
the surface of the water to maximize oxygen transfer with the atmosphere.
Lighting
The lighting you used for you freshwater system should also work for a fish-only saltwater tank. However, if you want to keep invertebrates, you will need to upgrade (more that just your lighting).
Nutrition
One part of a freshwater system that needs to be replaced is the food. Marine fish need varied diets. You need to supply your fish with a combination of fresh, frozen and live food. Flake food, although adequate, should not be the major portion of your fish's diet.
Changing the Water
Finally, when you are ready to make the switch to saltwater, you really should replace all the water in your system. It is best to start with nitrate free water to minimize the potential for algae problems. Also, many people think that adding salt to a cycled freshwater tank will yield a cycled saltwater tank. Experience have shown this is not true. Saltwater nitrifying bacteria are different than freshwater nitrifying bacteria, so they must be cultured from scratch. As a note, nitrifying bacteria seem to be pH and temperature sensitive. So moving some gravel from a warm saltwater tank (~85F/24C) to a temperate saltwater tank (72F/21C) will shock the bacteria enough to nullify any advantage from using the gravel (e.g., to shorten the cycle time).
