A Twenty-Gallon Tank Holds Sixteen Gallons in Britain

Nobody fills a tank to the top

A few centimetres of air below the rim are not optional: the water line has to sit under the bracing bar, the filter outlet needs somewhere to break the surface, and a tank filled level with the glass spills the moment anything is lowered into it. Stopping five centimetres short on a 40 cm tank removes 9 litres, an eighth of the nominal volume, and the substrate and rockwork take more. Any calculation that depends on the volume — a dose of treatment, a heater's wattage, a stocking rule — should use the water that is actually in there, and the honest way to find that number is to fill it with a measured container once and write the result on the cabinet.

Volume grows as the cube, and so does the weight

Doubling every dimension of the 60 × 30 × 40 tank to 120 × 50 × 60 does not double the volume; it takes it from 72 litres to 360, five times as much. Water weighs a kilogram a litre, so that second tank puts 360 kilograms on the floor before the glass, the stand, the substrate and the rocks are counted — comfortably more than most domestic furniture is designed for, and concentrated on a footprint of 0.6 square metres. The point where an aquarium stops being a decorating decision and starts being a structural one arrives sooner than the catalogue suggests, and the cube law is why it arrives so abruptly.

Frequently asked questions

Why are there two gallons at all?

Because Britain redefined its gallon in the nineteenth century and the United States kept an older measure. The imperial gallon was set as the volume of ten pounds of water, while the US gallon descends from a wine gallon that predates the change and was never revised. Both survived, and both are still in daily use in their own countries, which is why any recipe, fuel figure or tank size quoted in gallons needs its origin attached. Litres carry no such ambiguity, and a volume written in litres converts cleanly into either gallon whenever someone actually needs one.

Does the shape matter or only the volume?

Both, and they answer different questions. Volume decides how much water there is to dilute waste and how much a treatment dose has to cover. Surface area at the top decides how fast oxygen gets in and carbon dioxide out, and that scales with length times width rather than with the total — a tall narrow tank and a long shallow one of the same volume are not equivalent, and the shallow one exchanges gas far better. It is one of the reasons the old rules of thumb expressed in gallons per fish work badly across differently shaped tanks, and why a tank's footprint is worth quoting alongside its capacity.