Reef safe is a distinction used in the saltwater aquarium hobby to indicate that a fish or invertebrate is safe to add to a reef aquarium.
Every fish has a personality, is different, and, in some cases, are opportunistic feeders.
Tangs, which by most accounts are reef safe, may in adulthood eat some crustaceans shortly after they molt.
Many larger predatory fish, for instance eels and pufferfish, will adapt very well to a reef tank and will be problem-free as long as they have sizable tank-mates and no crustaceans.
Some aquarists have also had success in keeping smaller fish with predatory ones in reef tanks by adding the smaller fish at night, sometimes with newly rearranged rockwork.