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What kind of shark, what size tank, and what other fish are in there?
It sounds like the only thing you can likely do is remove the shark to another tank or return him to the LFS. Many "shark" type fish do become mildly aggressive as they mature. In most cases, this type of fish won't lose its aggression by adding more of its kind (generally loners and aggressive to conspecifics) or rearranging the territory in the tank (like you can do with some cichlids).
i have a 20 gal tank with three danios three tetras one angel and the shark i am moving my tank soon so i think i will try re aranging it then if it dosent get better i will return him