One of what kind of fish?? C'mon, you can be a little more discriptive than that. My RTC swallowed my crystal ball so, it's hard for me to devinate a correct answer.
With alot of species, the way to house them for life is to provide an aquarium with a length, at least, twice the fish's adult length and a width of 1.5 times the adult length of the fish. But, this general rule of thumb can change quickly if the species in question has a rigid inflexible body as an adult. Or, requires larger spaces to allow for a faster swimmer or increased DO to respiratory requirements.
Next time, get the name of the fish so we can provide a less broad-based answer.