Statistically, fish will survive longer in a well-maintained than in the wild, despite what you may have read in the press (P.E.T.A. and Snorkel Bob, on Maui, spring to mind).
Predation by other fish, not disease, famine, or old age, is the leading cause of fish death in the wild. Because of the vast extent of the seas, establishing a decent "plague" of any type is quite difficult. Because of the immensity of the ocean and the fact that nature has supplied a lot of "parasite pickers" and predators to take care of most issues in their infancy, you seldom witness a genuine epidemic of parasites or any illness in the wild. When a fish slows down in the wild due to sickness or old age, it is eaten.
Aquariums, on the other hand, are a completely other story. The great majority of salt aquariums are "closed systems," meaning they are not open to the ocean and rely entirely on human intervention to maintain the organisms alive and healthy. The fish population suffered greatly during the early stages of due to a lack of understanding of what the fish and invertebrates required. The fish did not survive for long. However, the science of has advanced significantly in recent decades.
All that being said, fish, invertebrates, and corals do die in marine aquariums. The major causes of fish dying in aquariums are the following.
Starvation and Diet
For two reasons, newcomers are far more likely to refuse hand-fed items. To begin with, many of the fish were not adequately fed from the moment they were collected until they were placed in their tank by the aquarist. Collectors seldom feed the fish they catch because it causes the fish to defecate in the shipping bag. Ammonia in the water equals poop in the shipping bag, which means less O 2 in the bag and burned fins and gills.
Sick fish, for the most part, do not eat. Furthermore, many aquarists do not provide the fish with the food that they require. Mandarinfish are an excellent example. In the wild, mandarinfish consume copepods and amphipods. They're not going to be enticed by that flake meal you're offering. There are ways to get picky fish to eat, but that is a subject for another essay. When a fish hasn't eaten in a while, they lose their appetite and it might be tough to entice them to eat again, even if they are offered a tasty treat.
Many aquarists do not properly acclimatize their fish. They may adjust them to the temperature difference between the bag water and the aquarium water, but they do not adjust them to the pH difference. Some fish (as well as many invertebrates) are more sensitive to changes in pH than others. Many creature fatalities in aquariums may be avoided by taking the time to gently regulate the pH.
Parasite and Disease
Parasites (such as and cryptocaryon) are carried by many fish, notably surgeonfish. Stress from capture and shipment might create an outbreak in your tank soon after you put them in your tank, even if it isn't visible when you receive them. Fortunately, most fish infections are treatable if identified early and treated appropriately. Before introducing new marine fish to your main tank, quarantine tanks are strongly advised. The parasites are not the "cause of death" in and of themselves. The true cause of death is generally asphyxia caused by the mucus that the parasite bores into the gills, causing the fish to suffocate. Even if the parasites themselves are eliminated, the ensuing sores frequently get infected, ending in death.
A Volitans Lionfish will consume every fish it can get into its mouth, no matter how much you desire otherwise. Although it may seem like an extreme example, many other fish species will simply not get along with certain other (or even their own) species. Before you buy a new addition, use a decent compatibility chart to discover what would most likely not work together in your tank. This will save you a lot of time and money.
Poor Water Quality
Fish require a steady habitat with specific characteristics in order to survive. Proper salinity, pH, low to no ammonia, and nitrites are among these factors. Immunological weakness, infections, and overall ill health are all linked to poor water quality. Fish that are weak are easy prey for other fish. Partial water changes, adequate filtration, the addition of trace minerals, frequently balancing the pH, and avoiding overstocking the aquarium are all ways to maintain good water quality.
Beginner aquarium owners, understandably, lose more fish than expert aquarium keepers. There is a lot to learn, especially with saltwater aquariums, and the learning curve is high to begin with.
It's worth noting that inadequate catch and shipping techniques are the root of many issues in hobby (and other) marine aquariums. A fish has gone through a lot of hands and been in a lot of shipping bags by the time it gets in your LFS's display tank. Typically, the collector collects the fish, transports it to a shipper, who bags it and delivers it to a trans-shipper or wholesaler, who enters it into his system, re-bags it, and finally transports it to your LFS. Knowing what to look for when buying fish can assist you avoid acquiring a specimen that could cause future difficulties.