Marine fish have worked out how to stay alive for millions (billions?) of years in the oceans. They just would not be here now if they hadn't. Each species evolved a means of surviving, whether it was a highly effective defense mechanism (such as the Volitans Lionfish's poisonous spines), schooling (the "safety in numbers" principle spreads the risk of an individual being eaten), the ability to hide from its pursuer (in rocks or corals), symbiotic relationships (the Clownfish wouldn't exist without anemones), or simply the ability to flee.
When a fish is taken from the ocean and placed in a confined environment, such as a home aquarium, its ability to run or hide from predators is drastically reduced. Simultaneously, it raises the level of competition for whatever food is available.
The chart below shows which fish can and cannot "usually" coexist in a closed area. In many circumstances, it also suggests which species will cohabit with care. Nothing is certain. Any generalization will always have exceptions, but the chart will offer you a place to start when determining what will work in your tank.
Saltwater Aquarium Fish Compatibility Chart
Take a look at the below saltwater aquarium fish compatibility chart to check if your fish are compatible together in the same aquarium.
Why Aren't These Fish Compatible?
For a variety of reasons, certain fish are incompatible, but it all boils down to competition. To stay alive, consume, and reproduce, everything in the water is battling for something. Predator/Prey, Territory Protection, Mate & Status Protection, Spawn Protection, and Opportunistic Feeding Behavior are the five fundamental forms of competition.
Predator or Prey
Food organisms are viewed as the victims of violent behavior, such as a swallowing an ornamental shrimp or a consuming a little Damselfish. This category pretty much speaks for itself. Keeping larger predatory fish with anything tiny enough for them to recognize as food is obviously not a good idea. Many aquarists maintain these fish in a predatory tank community with other predatory fish such as big Groupers, Hawks, Snappers, and other predatory species.
Others of the same or similar species, such as a young Angelfish and Jewelfish assaulting others, are the victims of this sort of aggressive behavior. Most fish behave in the same manner when it comes to territorial aggressiveness, especially if you already have fish in an aquarium and then add a new tank mate later. When you introduce all new fish to an aquarium at the same time, there are certain to be some conflicts until territories are formed. The fish normally calm down after that and life goes on. When you introduce a new fish into an established aquarium community, the problem of "harassment" is likely to arise, and it doesn't appear to matter what type or species of fish it is.
Mate and Status Protection
Other fish of the same species, such as a married pair of Clownfish assaulting others, are the victims of this form of aggressive behavior. Most animals display this sort of behavior, which is fascinating. If you place a married pair of Angelfish, Butterflyfish, Boxfish, or just about any other mated species in a tank and then add another male or female of the same species later, the same sexed fish will often go after the newly introduced sexed fish. It's a good idea to keep a tank to one mating species.
All other fish around a nest location, such as Damselfish defending their spawn from other fish that may wander into their nesting region, are the target of this hostile behavior. It makes it easier to keep fish that reproduce in captivity in larger aquariums with plenty of room for not just the nesting species, but also the rest of the tank's residents. This sort of behavior is not seen in pelagic or "free-spawning" species like the Yellow Tang.
Opportunistic Feeding Behavior
All other fish and invertebrates are the victims of this sort of hostile behavior. Triggerfishes are an excellent illustration of this habit, as they will consume almost everything. Fish with this sort of behavior should be maintained in a tank with other aggressive species. Triggers are frequently kept alongside other Triggers, as well as Lionfish, Groupers, Hawkfish, Snappers, and other bigger predatory species.
Before adding any additional livestock to your tank, learn about their specific behavior patterns to minimize competition, which might result in the loss of some of your prized fish and/or invertebrates.