Can I just add the gallon per inch of fish is completely out dated and should not be used! When following this rule it would mean you could have a 10 inch fish in a 10 gallon tank which is just ridiculous!
It is very misleading and if not clarified can indeed end up with ridiculous results.
In my mind 1 gallon for one inch of fish means that a 1" fish would at all times have a gallon of water to swim around in all by itself.
So a 10 inch fish needs 100 gallons all to itself at all times.
With imaginary borders, of course.