| A saltwater aquarium can be a fish only set up, a reef set up or a live rock set up. All are considered saltwater, the only differences are what you put in it.
Different fish are compatible with different environments.
Check out the fish you like, ones that would be suitable for a 30 gallon set up, and see what kind of environment they live in to help you decide.
The biggest thing when converting from freshwater to salt water is different filtration, substrate, and possible lighting and of coarse the fish.
Not to discourage you, but
one thing you should probably know is that smaller saltwater set ups are much harder to keep the water stable in and less forgiving if salinity or toxin levels are off.
They are also quick to build up high nitrate levels. This causes a need to preform more water changes to keep toxin levels in the safe zone than you'd have to with a freshwater tank..
I've talked to several people who loved it this way and enjoyed having a smaller set up and was very successfully with keeping them.
I have also talked to people who found all the extra work not worth it.
This would completely depend on your own preference and how much time you have to spend maintaining it.
Good luck in what you decide. | 
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