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Step 1
Choose similar-sized fish. As a general rule, larger fish eat smaller fish. If you don't want this to happen in your aquarium, only put similar-sized fish together. Remember, fish continue to grow throughout their life and can change dispositions in adulthood.
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Step 2
Notice attitude. Some fish are very territorial species and are meant for one-fish tanks. Tiger Oscars, for example, cannot live with fish of different species due to their aggressive nature. Other types of fish get aggressive during feeding or mating times and tend to pick on more peaceful species. Groupers are usually fine with other fish similar in size, but they tend to fight with other Groupers over territory. Archer fish are calm natured and get along with just about anybody. Talk to your dealer about what fish are most compatible with others and keep a sharp eye on any new additions.
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Step 3
Choose fish with similar diets. It is difficult to feed specialized diets to fish living in the same tank. Find fish that eat similar things (such as aquatic plants, bloodworms and coral) and, of course, won't eat each other.
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Step 4
Look for physically dissimilar fish. Fish that can be mistaken as the same species may get aggressive toward each other. The "there is only room in this tank for one of us" mentality takes over.
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Step 5
Choose unrelated fish. While single clown fish can be relatively peaceful, mated couples tend to gang up on other fish in the tank and often become territorial especially when introducing another male of the same species.
