If you're thinking of getting a Marine, or Saltwater aquarium, you should know that generally you have fewer fish than in a freshwater tank because the many marine species tend to grow bigger in size and require more space in their 'ecosystem'.
Marine species are often caught in the wild, although these days many are bred specially. Captive bred species, generally speaking, will be a bit hardier than species caught in the wild so they'll be slightly easier to look after. The very large amount of stress when a fish is captured in the wild can often weaken them when they are then introduced into an aquarium. Often, it can be days before they start eating.
Consider, if possible, making your purchases from retailers that deal with breeders, or if possible buy directly from the breeders themselves.
Essentiallyl, there are five different types of saltwater aquariums.
1) Fish only aquarium.
Probably the easiest to care for because you only with one type of species (not one species). Amongst the saltwater fish there are both tropical and coldwater types. For obvious reasons you don't keep them together! The water temperatures vary greatly. Tropical fish are usually brighter in color, and therefore more appealing to aquarium lovers than the fish available in the cold water variety. Most people are somewhat familiar with a few tropical fish such as the Clownfish or the Angel fish. However, few people have heard of such cold water varieties as, the Shanny or the Tompot Blenny.
2) Invertebrate only aquarium.
Usually consisting of shrimp, prawn, hermit crabs and perhaps even starfish or sea cucumbers.
3) Fish and invertebrate saltwater aquarium.
Naturally the greater complexity of interplay between the species you have in this kind of set-up makes it more difficult to maintain than either of the two categories. For example, some invertebrates feed on certain fish - the reverse is also true that some fish will eat your invertebrates. It's vital to do some quality research and make sure not to put predator and prey together - if you do put them together you'll burn through money and livestock in no time at all. Diseases can also spread more rapidly and are more difficult to prevent and cure in aquariums containing both invertebrates and fish species.
4) Coral reef aquarium.
Reef aquariums have a reputation of being tricky to maintain so it really pays off to do your research thoroughly before you think about setting one up.
5) Speciality aquarium.
An example of a speciality aquarium would be an aquarium full of sea horses. Sea horses generally are best not kept with other types of fish or marine life, since they're timid and slow eaters. Their food source could very easily be taken away by other inhabitants.. Seahorses prefer to swim vertically so ideally they need a taller tank. Octopus and a shark or a ray are other examples of fish that have special needs - they would definitely fall into the category of speciality aquarium.
Whatever type of aquarium you decide to set up, it can't be emphasised enough that you should so some good research before you ever think about buying your lifestock. Not only will it save you money in the long run, it will help avoid the awful situation of buying livestock only to see it die. As a responsible fishkeeper you'll want to avoid that at all costs.