What Else To Add?

[Corleone]

It's quoted widely here in the US, and always without qualification, so I've always assumed it was US gallons.

Converting, 1 in/USG=1.2 in/UKG, and 1 in/UKG=0.8 in/USG. That makes a difference of 10 inches either way in my tank.

Still, I think the point of the rule is more to put a slightly underestimated but easy to follow rule of thumb for new fishkeepers. I've regularly seen people on this forum say that with experience and a mature tank, 1.5 inches/gallon or 2 inches/gallon are acceptable.

 

[OldMan47]

Its US gallons and UK gallons both. Its not really a rule but a guide designed to keep new fish keepers out of trouble while they learn to judge how many fish is too many for a tank. The only time its close to accurate is with small fish, less than 2 inches full grown, in small tanks. After that tank size, if you don't develop a sense for population density, it will keep you out of trouble but will also keep you lightly populated. If you use it for large fish, it will get you in trouble fast as the fishes body size, I think of it as volume, better reflects the biological load they put on a tank and that increases as the cube of their length but not quite because the body size to food needs ratio changes as the fish get bigger.

 

[mg-guner]

5 tiger barbs
4 green tiger barbs

2 silver dollars
3 peppered corys
2 bronze corys

1 redtail black shark
1 raphael catfish (talking catfish)
1 plec

thats all for me folks, thx