The larger the saltwater volume the easier it is to keep water chemistries stable, and that usually translates in fish lingo as "easier to maintain." 30 gallons is way small for a beginner to the marine hobby, but it is not impossible.
Smaller tanks require a lot more reseach and a lot more maintance. Here is why:
1) Smaller tanks require more frequent water changes to keep them clean and stable. This could amount to 4 gallons 2x a week on a 30 gallon tank, including gravel vac and filter cleaning depending upon the fish species.
2) They do not have enough volume for dilution, small changes to the tank (temperature, chemical, salinity etc) mean major stress on your fish. Salt makes any chemical placed into your tank 10X more toxic to the fish than if the same amount were placed in a freshwater tank, that includes ammonia, nitrites, and nitrates.
3) Most marine fish, when properly cared for and thriving, will outgrow a small tank very quickly.
4) Most marine fish, along with their larger size, have larger attitude problems, which means battles galore if thankmates are not choosen carefully or decor not properly arranged.
5) Most LFS employees know next to nothing about marine aquarium maintance let alone about the specific biology of each fish they sell. So that leaves a lot on the shoulders of the would-be marine aquarist to learn on their own. Marine fish are expensive, so trial-by-error is strictly prohibited against.
But if you are truely dedicated to your 30 gallon marine tank, go for it. I too have had various success with yellow tangs in such size tanks. Sometimes it works, sometimes not. Yellow tang do get very large, they are also a very nervous, swift swimming fish. In small tanks they tend to bash themselves against the glass every time someone in the room sneezes, and this is definately bad for their health. More than one yellow tang in such a small volume is to spell disaster since they will battle each other to the death. I have even seen yellow tangs go at each other through glass tank seperators. Even though they couldn't get at each other, it still drove them crazy to be within sight.
~~Colesea