Realistically in noob fishkeeping most people would probably tell you that you could keep one in a 75g because they really won't get over 20" anytime soon and most don't get over 18" or so in the aquarium. Well trained fish keepers know they need space to roam(and although it may take a long time talking 5 years+) there is a small chance it could get over 18" eventually so that's why 180g is suggested.
So you don't need a monster tank to keep them especially if you're in some small town somewhere and with no one to tell you better.