Livestock Inspection
Disease
A trained eye can spot disease, which needs to be treated in the early stages before it gets to an advanced stage or causes death of livestock and contaminates other inhabitants of the tank.
In a large tank, it could be necessary to strip the whole tank to catch an individual fish if the fish can't be treated in the tank or is suffering from attacks by other tank inhabitants.
Treatment
Treating disease in a reef tank can be difficult as the invertebrates can be riled by most chemical treatments; there are other means of treatment however.
Compatibility
Compatibility of fish is an important factor, good knowledge prior to adding new fish to an aquarium is essential, fish can be purchased on impulse or looks and when added to a tank and not survive or cause damage or death to other inhabitants. The order in which fish are introduced is also important, occasionally a hostile rogue fish will need to be removed and re housed if it attacks other tank inhabitants.