Instructions
Choose fish that are about the same size. Don't pick fish that will grow large and dominate the tank, potentially turning into a predator of the smaller fish. Large fish also cause aquarium capacity problems. If you want a fish like this, put it in its own aquarium where it won't bother other fish.
Don't put overly aggressive fish in with other fish. Some fish should be in their own tank away from other fish they can harm. Aggressive fish include snappers, triggerfish, damselfish and scorpionfish. You should avoid even putting mildly territorial or predatory fish in the tank as well, because they can turn aggressive toward other fish.
Put plenty of docile fish that can live together in small groups in your saltwater tank. Good examples include clownfish, seahorses, file fish, mandarinfish, surgeonfish and anthiinae. These fish add plenty of color to your tank while cohabitating peacefully with each other.
Talk with your dealer about the behavior of the fish you are interested in purchasing. Even docile fish can have a few bad apples that act aggressively and would not make a good fit, and your dealer should know if a fish is causing problems. Ask your dealer for advice on what fish should be combined or not combined.
How to Choose Compatible Marine Fish
Getting a big saltwater tank and filling it with marine fish can make an exciting addition to your home, but you need to keep in mind a few realities of having different types of fish in the same tank. Some fish are aggressive and should have their own tank, while some fish live well together, so you will need to follow a few steps to make sure you're creating the right environment for your fish.