While most freshwater aquariums use a fairly standard set of equipment the larger array of organisms available for the marine tank can make a difference in just what equipment is needed.
If you've ever seen a beautiful reef tank with clownfish snuggling in their anemones while shrimp and crabs hunt for tidbits in the surrounding rocks and fish so brightly colored that you think they must need batteries cruise overhead it is easy to understand why more and more people are choosing a marine aquarium instead of a freshwater tank. A properly set up and smoothly operating marine aquarium is truly a breathtaking thing to behold.
However, the marine aquarium is also more complicated, expensive and time consuming to set up and maintain. Most marine aquarist start off as freshwater fish keepers and move into the hobby once they've successfully gotten their "feet wet" taking care of a freshwater tank.
There are a few benefits to this approach some of the equipment can be converted to a marine environment is one. Another is that they probably have at least a feel for water chemistry and how it can affect the inhabitants of an aquarium. The biggest benefit though is that they also realize that like any other pet it takes time to maintain them.
However while there are some basic concepts that apply to both types of tanks a marine aquarium is a very different kind of beast than a freshwater tank. The chemistry is more complex, there is additional equipment required and the inhabitants often have very different needs than their freshwater brethren.
A lot of these differences depend on what type of marine aquarium is being set up. There are three basic categories of marine tanks the fish only tank, the fish and live rock tank and the reef tank. The decision on which type to set up is going to dictate what kind of equipment is needed, what kind of inhabitants the tank can support and the initial and ongoing cost of maintaining the aquarium.
The fish only tank usually cost 2-3 times what a comparably sized freshwater tank cost, add live rock and the cost goes up to 3-4 times and a reef setup usually starts at 5-6 times what you would spend on a basic freshwater setup. Maintenance time follows the same curve with fish only tanks taking about double, add live rock and add some more time and if you choose a reef setup you can be spending more time, at least initially then you spend with your spouse. That being said these extra outlays also result in a display that most freshwater tanks can't even hope to compete with.
A marine fish only tank requires the least additional mandatory equipment and supplies. At a minimum there is the salt, a hydrometer to measure it and some new test kits. Realize that this setup is not suitable for all fish. A fish and live rock setup will require a couple of extra test kits, some additional pumps to move the water and probably a protein skimmer and additional lighting. Set up a reef tank and you need a few more test kits, additional lighting and a protein skimmer become an absolute necessity as well as a smaller secondary tank for keeping or culturing live foods for the corals and fish.
All that being said one has to wonder "is it worth it?" well the truth is that if you fell in love with that cute little clownfish or the liquid rainbow of a reef tank than a goldfish or a few guppies just aren't going to cut it. One way to help avoid costly startup mistakes and to take extra time out of the equation that is becoming ever more popular especially with reef tanks is to hire a professional to set up and maintain the tank.