A common misconception is that you have to graduate from freshwater to saltwater. This is of course not true, as a freshwater species aquarium can be as challenging as a reef set-up, or as easy.
Knowledge is once again the key to a successful start and the more information you have the more fun you will 
get out of the hobby.
Another fairy tale is that saltwater requires a larger tank than freshwater. This is not so, a larger aquarium is generally easier to maintain than a small tank. Just picture nature as the ultimate Eco system. Any aquarium you set up is a copy and the smaller the copy the more difficult it gets to maintain and to provide a healthy and balanced environment.
Saltwater aquariums do not have to be expensive either. Despite the fact that most saltwater species are on the more expensive side, it does not imply that you can’t start out with "cheaper" fish.
The more you get into additional equipment the more expensive it gets, but the same applies to a freshwater aquarium.
Freshwater and saltwater, including live rock, can be compared by size and cost. The exemption is a reef set-up, where the lighting system alone can easily cost as much as a fish only tank.
As you are fired up by now, we will take a look on what equipment you can use in converting a freshwater into a saltwater aquarium.
The Aquarium
The tank itself can be used as is. There are no special aquariums that differ in fresh or saltwater set-ups. It is advisable to clean the tank with a sponge or filter floss with water only. Household chemicals do contain substances that are detrimental to the system.
Filtration
The original filtration can be used in the new saltwater aquarium as well. The beneficial bacteria are the same and work as well in salt as they performed in the freshwater aquarium. Later on in time, especially if live rock and more animals are added, a change in filter type might be justified. In this case a trickle filter or a protein skimmer can be added.
One exception would be the undergravel filter. As explained further down, sand is the preferred choice of a saltwater aquarium and this kind of filter will simply clog up.
Pumps/Tubing/Heater
The tubing, heater, and air pumps from the freshwater tank can be used in the new saltwater tank. If cleaning is needed, water only is the way to go.
Ornaments/Décor
Plastic plants and décor should not be used in a saltwater tank. Most saltwater fish nibble on everything they can get a hold on. The new decoration can include live rock, rocks, or fake corals in the beginning.
Substrate
The substrate for saltwater should either be sand, crushed corals, or aragonite. The substrate in saltwaterdoes not only provide biological filtration as in freshwater, but also as a home for many animals that might be added over time i.e. crabs.
Crushed corals and aragonite will keep the pH stable as well as provide calcium for a later update to a reef system.
Should sand have been used in the freshwater set-up, it can be used for the saltwater tank.
Lighting
At this early stage the regular fluorescent light will be fine. To enhance the color of the fish, one might be replaced with an actinic bulb. Lighting will get more important with a reef tank later on if desired.
Test Kits
Freshwater test kits can not be used in saltwater and have to be replaced by saltwater test kits.
General Notes:
In addition to the above, a saltwater mix and a hydrometer to measure salinity will be required. For the first batch of saltwater, fresh water should be used instead of just mixing salt into the tank. The maintenance schedule for cleaning the tank and water changes remains the same as in freshwater.
For most tanks, all there is to it is to replace the gravel, remove and replace the décor and to add water with the seawater mix.
Most of the additives can be used as well, such as a water conditioner and Algone.
Some additives are specifically made for saltwater, so check the labels on the packaging before using them.
Any products that contain copper must be avoided. Copper is one of the biggest enemies for saltwater aquariums.
After the conversion, the tank will still cycle. But if the existing filtration, with the bacteria cultures in tact, was used, the tank will complete cycling much faster then a new set-up.
