If you keep a cat in a cardboard box for life it will still live but will it ever be happy?
What you must consider is the environment of the fish, where they come from and how they react in confinment. Tangs NEED space, 50 gallons just doesnt cut it. With the exception of perhaps a Kole/Yelloweye tang i would not even consider keeping them in under 100 gallons.
Yes its true that they are often kept in 50 gallons but i have seen yellow tangs grow up to 9" across and IMO that is far too small for a fish with this growing potential. Lets also consider the behaviour of tangs, they are extremly terriotorial and aggressive, once its established its territory then you wont be allowed to put another fish in there that remotely resembles its shape colour or dietary requirements.
In the wild, fish like Clowns, dwarf angels ect have relatively small territories, this can be measured realistically in terms of the size of our home aquariums and this is why many of this fish aclimatise so well as they wont travel much further than this in the wild anyway. Tangs are a different story alltogether. They have territories (well not territories really but more like grazing patches) that stretch for miles as they cruise above the reef looking for algae to eat. When these fish are confined to a water filled box their temperment changes and a naturally shoaling fish suddenly turns aggressive and will not tolerate close relatives near them, (not saying that more than 1 tang cant be kept in large tanks but its damned hard as i have discovered from experience).
I can see the attraction for tangs in smaller tanks, they are brightly coloured, active fish but sorry, i just cannot agree that a 50 gallon is going to make a fish like this happy. If you want a fish more suited to this size of tank then buy a dwarf angel, whatever colour you want to add in your tank from a tang can be matched with a dward angel...
Yellow tang/Lemonpeel angel/Heraldi Angel
Purple Tang/Coral Beauty
Chevron tang (Juvenile)/Potters angel
And so the list goes on.
Perhaps a Kole tang will thrive in 60-70 gallons as these grow smaller and in the wild they adopts small grazing patched much like Powder blues. (Dont even attempt a powder blue as they are evil SOBs and definately need more than 100 gallons)
If you choose to ignore this advice then of course this is your choice but i will wanr you now, life on any marine fish forum will be pure hell as experienced marine aquarists will have their say and its usually very direct
Places like live aquarium.com are just plain wrong and need to update their advice. Its ok to sell small tangs and yes they are fine in small tanks when they are small but 2 years down the line (usually much sooner) the fish will outgrow its tank.
Lastly...
Tangs are easily stressed, they are aggressive too which doesnt help the situation. An easily stressed fish like tangs are whitespot magnets. This means that your tang in a 50 gallon tank will be under continous stress \nd you will be fighting to keep whitespot at bay for years to come. These fish can potentually whipe out your entire tank of livestock and yet live themselves. I know this from experience.. i had a large stock of fish and in the last 12 weeks all i have left is a sohal tang, 2 emperor tangs and a potters angel. They stress induced disease they brought killed the following fish...
BLueface angel, Asfur angel, Comet, Poison fang blenny, Royal gramma, silty wrasse, 6 line wrasse, achilles tang, powder blue tang, powder brown tang, goldrim tang, 2x bluethroat triggers, Anthias and a few more that i cannot think of at the moemnt (sorry im rushed for work).
All this happened in a 260 US Gallon system and even in this size they fought among themselves to the point that stress weakened them to catching parasites. and then the disease wiped out the rest of the tank. I now have the fish in quarentine a\nd the main tank is half full with water so the corals can survive. The outcome now is that these tangs now live in copper to eliminate parasites, the main tank must remain fish free until the parasite is dead. September is the date my fish can return to the main system.. So if you are prepared for all this then feel free to ignore this advice, but if you value the views of people who have had 1st hand experience with these beautiful fish then you will either get a bigger tank or choose an alternative fish