I have always wondered who makes the rules and more so HOW?!
The fish make the rules - no question.
We're merely the idiot groundskeepers. We do best when we don't forget that.
Problem is we can't telepath with them (yet!) to get their requests, and what we have instead is a hodgepodge of people's experiences in keeping these fish at home as well as what we know about their behavior in the wild. Lots of good information, but it only amounts to rules of thumb, and little more. This can make it tough on a newbie, but doesn't make the info useless!
If you have lots of tolerance for risking your fishes well being and don't mind replacing dead fish, then you generally disregard these rules of thumb and end up with a crapload of big fish in a "big" 75 gallon or "even bigger" 90 gallon tank. Hah! With only 6 square feet of swimming space - minus filters, drains, rocks and coral - both of those tanks are equally small to the fish.
I'd be looking for something with around double that space.
Sadly, many folks like to pull what amounts to a "stunt" in their tank and just try to keep as many fish in it as possible, or see how many big fish they can keep, etc. Certainly with skimmers, carbon and GFO, refugiums, bio-plastics, etc, etc, etc, etc it may be possible...for a time....
until something happens and the stunt finally fails. Why try to play know-it-all and Superman with your tank all at the same time??
To put a nice, fine point on it....setting a tank up in this way amounts to being in denial about the amount of time our fish should be able to live if allowed to live relatively natural lives.
In a 20+ year life-span, sooner or later something
is going to happen. When a tank is so maxed out like this (or if you prefer, your fish are so limited like this), you're just waiting for the proverbial straw that broke the camel's back. You've eliminated almost all your margin for error and you're much more likely to be looking at a dead animal or even a crashed tank instead of just a broken pump, behavior change in a fish or whatever it is that finally happens.
I'm not a member of the Tang Police, but I support their cause.
-Matt
P.S. You mentioned that you have a 120 gallon tank. Almost all rules of thumb in the hobby (everything from watt/gallon lighting to inch/gallon fish stocking) are anchored to traditionally proportioned tanks - this includes the recommendation for tangs. In a nutshell, "tradiational" means (e.g.) 10, 29, 38, 55 and 90 gallon tanks as well as the common variations that aren't as tall. A 120 gallon tank has significantly more (~30%) swimming area than the typical 4 ft tank, so I wouldn't feel nearly as bad seeing a tang or two (consider the Bristletooth family first....some are often found solo in the wild....not dedicated schoolers) in a tank that size - especially if the rocks are minimal and laid out for maximum swimming room. Bigger than 120 is still going to be quite a bit better with almost all tangs.
P.P.S. I presently keep three 1.25" Barnacle Blennies and nine .75" Sexy Shrimp (along with a crapload of SPS coral) in a 50 gallon (4.5 square feet). In my freshwater days I kept a single Oscar and single Pleco alone in a mostly bare 75 gallon tank. These are representative of ideal fish stocking rates, IMO. (The Oscar and Pleco both lived more than 20 years.)
