One way of thinking about it is to have no more than 1 inch of fish per gallon...
That old wanker is still making the rounds?
Let's think about it.
"One inch of fish"...is that a linear measurement?
Lengthwise or heightwise? Hmmmm...
One inch lengthwise of a khuli loach, and one inch lengthwise of, say - Marge?
Are they comparable? You can't even
get one inch heightwise of a khuli...
Seriously. The ole 'one inch per gallon' thing is problematic on so many levels I can't believe it's still being considered. To start with, fish are three dimensional.
How about one cubic inch of fish? Will that work? How do you measure a cubic inch of fish?
Sheesh.
Things to consider when stocking a tank:
Adult sizes of the fish you house - you do expect them to become adults, right?
Waste production and the filtration to handle it.
Oxygen needs of the population.
Water movement (or lack thereof) preferred by your fish.
Territorial needs.
Environmental needs - do they need rocks, wood, plants, open space?
Social needs - are they schooling fish, loners, gangbangers or couples?
Swimming space - are they vigorous swimmers, or do they just hang out?
Compatibility of species - water parameters, aggression levels, feeding habits.
Consider all these things (and I'm sure you all can come up with others) and then factor in the size of the tank you have available, your ability to create and maintain a stable environment within it, and the interactions among the inhabitants.
THEN tell me how 'one inch per gallon' will help you to select fish responsibly.
OK. rant over. sorry all.