How many times do people want to argue over tangs? They are beautiful, awsome fish that have personality (Good and Bad).
How many grouper's are in a 55g, sharks in a 100g? Aligators in a 5x5 cage?

It's an endless battle.
It is only a myth if you dont look for the long term health and life of the tang. Or any animal for that matter.
Tangs swim all the time, and graze none stop. If you put the amount of rock they need in a 30g to feed off of, they would have no room to swim, stress out, get sick, and die like most tangs do.
Putting tangs in a 40g and maintaing zero amm. and such is possible for the advanced hobbiest if you have a big enough filter system and maintain the tank and such. I think the smaller tangs would be fine in 4 foot tanks. But you have to look at some tangs get to 8", thats only about 3 feet of swimming room (not 4') . That kind of setup should only be maintained by the most dedicated aquarist.
Most fish stores have all there tanks plumbed together, then going to a sump, and filter system, which means more water volume. They still dont have alot of room to swim, and arent in the healthiest condition.
Most people wont feed them accordingly to maintain them for long periods of time in a small setup, or over feed and dont stick with a water changing schedule. Then there wondering what happened? I dont understand! He was perfectly healthy?!?. Bottom line is most people shouldnt cram them because the majority of people are too lazy or too busy to keep up with the bio load.
Here's a so called myth. Fish grow to their environment. Alot of people will say its true, and I'm not going to argue with them. Feed sparingly so they dont grow too quick!??
People do it too all kinds of fish and animals. They either dont know better, or just dont care. Majority dont care.
Dont take this as you hit a nerve or I'm ranting
There are successful people out there that take great care of their animals and value there life. I applaud them. I'm just simply starting what has been stated over and over.