75 gallons is the minimum recommended for a yellow tang
IMO.
however if you find a small specimen you might be able to keep it for sometime in a standard 55 gallon tank which is 48" long
in a 50 gallon tank, thats only 36" you would be running more risk. not that you cant do it, but if you start loosing them to stress related sicknesses like Ick, etc.. its likely due to a tank thats too small.
personally, i would look at another type of fish, but thats just my opinion