I think you need to reread Rivercats' post @#5. He does mention a number of things to consider including fish weight and fish size.
My post also said that "common sense" needed to be used when stocking. ( But I'm not knit picking
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You make a good point regarding what type of gallon( U.S. or Imperial). I think most of the world would convert the volume into liters anyway so it might not be as big an issue unless you are in the U.S. ( We're behind the times, what can I say?
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I think it really boils down to: How much food goes into a fish and how much poop comes out of a fish to determine how much water is necessary to keep all that from creating a toxic environment.
I'm old school, I use my eyes
Then you need to take into account, how much do the fish swim to know if that volume should be in a long tank or a shorter tank. Then you need to take into account a whole bunch of other factors. This is why impulse buying of fish is such a bad thing to do. (And I used to sell them in stores so I know what I'm talking about.) I always discouraged this type of sale. It usually turned out bad for business.