Irensing, you are actually irritating me.
This is one of the most incorrect statements you could have made. Ill give you some facts...
A lot of fish die on the way from the ocean, the wholesalers and then to the shops. The next part is most shops move fish quite quickly. They are not kept in small tanks long and even then, my LFS has told me they receive tangs who are completely unable to adapt to life in tanks and die anyway. next, if I was to take you, and tell you to live in a closet for the rest of your life... could you do it? Yes, probably, but would this make you happy? Certainly not.
In fact, my LFS DOES keep its tangs in 100g tanks and nothing smaller. That is because he is responsible. Others are not this nice.
Also, increasing your water volume does not increase swimming space. For example, sea horses prefer a taller tank and will not do well in a short tank due to how they swim, they may survive but be unhappy. Tanks and angels need large amounts of horizontal swimming space and thus you could have a 125g tank which is narrow and small and have your tank die. So, if I add a sump, which adds water volume to dilute our errors, how does this create more swimming space that will allow you to hold more fish? you would still be crowding more fish to close together. Im having territory issues while adding a 4th fish to my 60g as my clowns take over there 1ft cube area and anything that enters gets bitten.
To your comment about knowing if they are stressed, a member literally suggested that due to the activity of his tangs in his 180g tank that his fish may need a larger tank. He has obviously stated that there are levels of stress shown in that fish or the suggestion that their may be as they continue to grow.
So before you know all the facts. Please do not state that an LFS is a perfect example of how a fish should be kept in the long term. They do what they can to sell you a fish and also do not take much regard as having the correct systems to house each and every large fish they get will cause the LFS to incur costs it simply cannot afford.