Where the rule came from and what it assumes
The inch-per-gallon rule survives because it is simple enough to say in a shop: measure the fish, add up the inches, stop at the gallon count. It dates from an era of small, lightly stocked tanks and rests on quiet assumptions — that fish are roughly interchangeable units, that waste scales with length, and that the fish you measure is the fish you will have. Every one of those assumptions is wrong often enough to matter, and the rule's failures are not edge cases. It is possible to stock a tank that obeys the rule exactly and is overcrowded, underfiltered and behaviourally miserable, and possible to stock one that breaks the rule and thrives.
Length is not load
Waste is what actually limits stocking, and waste scales with body mass and appetite, not inches. A goldfish and a neon tetra can both measure an inch and a half; the goldfish is a deep-bodied, heavy-feeding fish that produces many times the waste, needs far more oxygen, and will out-pollute a whole school of tetras on its own. A ruler counts them as equals. The same distortion runs through any comparison between slim and heavy fish — slender danios against plump platies, a pencil-thin kuhli loach against a juvenile pleco of the same length. Two tanks with identical inch totals can carry utterly different biological loads, and only one of them is being measured honestly.
The things a ruler cannot see
The rule also measures the fish in the shop bag, not the fish in the tank next year: a two-inch juvenile of a species that reaches a foot passes the test today and fails the tank by winter. Behaviour is invisible to it as well. A territorial fish needs floor space and sight-lines broken by decor, a fast schooling fish needs length to swim, and a schooling species kept below its group size is stressed at any inch count. Surface area matters more than volume for gas exchange, so tall and long tanks of equal gallons stock differently. Filtration, planting and water-change habits all shift the real capacity further, in both directions.
What to judge stocking by instead
Start with adult size, always, and with the fish's build and feeding level as your waste estimate — deep-bodied, heavy eaters count for far more than their inches. Then judge space by behaviour: swimming length for active species, territories for claim-stakers, group size for schoolers, floor area for bottom dwellers. Weigh the system honestly — filter capacity, plant growth, your actual water-change rhythm — because the tank's capacity is the system's capacity, not the glass box's. Online stocking calculators and experienced shop staff both work from these factors, which is why their answers differ from the ruler's.
A better question to bring to the shop
Instead of asking how many inches a tank holds, describe the actual fish: this species, this many, at adult size, in this tank with this filter. That question has an answer worth having, and it catches the goldfish-in-a-bowl absurdities the ruler waves through. For the combination written on your shopping list, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.