Why one inch per gallon fails
The rule says one inch of fish for every gallon of water. It is tidy, and it gets a surprising number of tanks wrong in both directions.
Length is the wrong measure
A fish's weight grows roughly with the cube of its length, so a fish twice as long is far more than twice the animal. Ten neon tetras and one oscar are both about ten inches on paper. One is a gentle cloud of tiny fish and the other is a messy, powerful predator that needs a tank of its own.
The planner scores load from a bioload weight for each species, softened so that big fish are not punished beyond reason. That is why it can say a tank is fine for a shoal and over for a single large cichlid.
Water volume is not the whole story
A tall, narrow tank holds as many litres as a long, shallow one, but a fish that swims lengthways has far more room in the long one. The planner checks the longest side against how far each species needs to travel, separately from the volume check.
Some fish want more than their body size suggests. Territorial species claim a patch, and a tank without enough floor space turns that patch into a fight.
The rule ignores who lives with whom
Shoaling fish kept as a pair are stressed fish, however much water they have. Fin-nippers ruin slow, long-finned neighbours. None of this shows up in a count of inches.
So the planner reports four separate answers: Space, Load, Water and Group. A tank can fit on one and fail on another, and blending them into one score would hide exactly the thing you need to fix.
What to do instead
Start with the tank. Use the tank volume calculator for the real water, then put your list into the planner and read each verdict on its own. If one is amber or red, the reasons under it say what to change.