Hi aj,
Give us some names (or pix), and we'll give you some answers! Is it a gourami? Or is it really a pacu? Or is it a pleco or a cichlid? Depending on the fish, you can try any of several options--upgrading to a larger tank; adding real/fake plants to give the smaller fish protection; moving the aggressive fish to another tank; returning the fish.
Often, this problem occurs with the dwarf or other gourami species. They are often touted as "peaceful" community fish, but this only applies to large tanks in which the fish are kept without conspecifics (other members of the same species). I've had this problem in my 10-gallon before, and ended up moving the gourami to my 55-gallon semi-planted tank. Though he and his other male dwarf gourami friend still get semi-aggressive towards each other, the other fish are not harmed.
How big is your tank, and how many fish in it? With this info, we can help you out.
Jonathan