That's what EAGLES is trying to say. Also, some fish command a larger territory and will kill anything that goes into it. Therefore, if you put X fish that can grow to adulthood without difficulty in say something like a 75g tank, but it's territory takes up the whole tank, then you have reached your limit with one fish. For example, if you have an adult Domino Damsel or Humbug Damsel alone in a 75g tank, I'm sure that they will command the whole tank. Thus, you have just bought a $5 fish that grows no more than 4", but will be your only fish. If you take that information, you can see that 1" per 19g. Yet, you can turn around and put something like a school of 7 chromis in there with not much of a problem at all. When fully grown that would be about 1" per 3g. Also, another factor is room to move. For example, tangs and surgeonfish are water column fish. They make mad dashes that last MANY yards and travel wide distances foraging for food.
Let's take a Sailfin Tang for example. They grow 15" long. Their fins fully expanded make it a good 20" tall. If you use the 1"/gallon rule, then all you need is something 30" long. That would be as small as a 29g tank. But because of their need to swim and they huge size, you need more. It is recommended by most to have at least 125g tank (liveaquaria.com). But, lets think of it this way, a Yellow tang is said to need at least a 4ft tank. They grow a maximum size of about 7". Now 48/7=~6.8" of tank per inch of fish. Use that info for an adult Sailfin and you get 102". Round that up to the nearest foot and you get a 9' tank. Now, that's for when it does dashes. But they are capable of dashing far beyond 9ft, so it needs room to maneuver and turn. Since it can't turn on a dime while dashing, it will need at least it's body length. Let's just say 2x it's body length for a quick, but casual turn in a dash. That's a 30" Wide tank. Now vertically, when it fully stretches out, it's a good 20". It needs room to move up and down, but like almost all fish, vertical room isn't too important. But he still needs room to go up and down. Let's take 2x it's total height. That's 40". So, now we have a tank that is 108"x30"x40" to properly accomodate an adult Sailfin Tang to it's death of old age. That's 560g! Not 125g like recommended. Of course, this is good news. In the mean time, the Sailfin isn't very aggressive and only takes up a space that is about 15"x20"x3". So, the other space can house many other fish of smaller size. Also, this huge amount of space will also probably provide the fish with enough food to sustain itself if there are no other competitors for its food source.
I guess all I'm trying to say is, not only is the inch per gallon rule completely worthless in both freshwater and saltwater, but even the recommended gallons are also a little off if your goal is truly responsible husbandry. That's just my opinion and theory though
Truthfully, I'm working on an article for one of our clubs that discusses tank size and fish species.
