How long of a tank for an "active swimmer"

[nomadman2003]

Reading on the Boesemani Rainbow I've learned they need room to swim. One site said 48" minimum, another said >30". My question: is there a good rule-of-thumb for a fishes body length vs. the length of the tank? Like 10x or 15x? Or does this vary too much on the specie? I've seen Barbs in the pet store move pretty dang fast.

Thanks.