Most freshwater sharks have a semi-aggressive temperament. They usually become territorial when introduced into a tank and lash out in case any fish tries to encroach. Besides, most species of freshwater sharks tend to view smaller fish around them as food and begin to treat them as prey. Again, not all freshwater sharks exhibit this behavior but do your due diligence before introducing them to a community tank. Also, most freshwater fish are quite large, so the space factor is also something to look into.