You should probably have some idea of what type of fish you want to keep before stocking the tank to avoid problems later. Do some research on each fish before adding them to your aquarium. Some fish are predatory or aggressive and so should be kept either by themselves, or with other predatory species. For instance a neat predatory aquarium can contain lionfish, eels, and groupers. Some fish are slow feeders, or are very shy and should be kept by themselves in a species tank.
Examples are shrimpfish and seahorses.
Tangs can become territorial and aggressive towards other Tangs that are added later. To avoid this you can change the decorations around before adding a new tang to an established aquarium. By most accounts marine systems need partial water changes on a regular basis. Note that this is higher than what is usually recommended for freshwater. In deciding how much and how often you wish to do water changes, keep in mind that for stability, smaller water changes done more often are better than large water changes done less often.