Aquarium Volume Glossary

A

Actual water volume

The water in the tank after subtracting the fill gap, substrate, rock and equipment from the geometric volume. The number dosing should use.

Aragonite

A calcium carbonate sand used in reef tanks. Fine grained with low porosity, so it displaces more water per inch than gravel.

B

Bow depth

On a bow front tank, the extra front-to-back distance at the centre of the curve beyond the flat depth at the corners.

Breeder

A rectangular tank with an unusually wide front-to-back footprint relative to its height, such as the 36 × 18 × 16 inch 40 breeder.

Bullnose

A tank with one semicircular end and one flat end.

C

Cubic inch

The volume of a one-inch cube. 231 cubic inches make one US gallon.

Cubic centimetre

The volume of a one-centimetre cube, equal to one millilitre. 1,000 make a litre.

D

Diameter

The full width of a circle through its centre. Twice the radius.

Displacement

Water pushed out of the way by anything solid in the tank. Substrate, rock, wood and equipment all displace water.

E

Exterior dimensions

Measurements taken on the outside of the glass. They include panel thickness and overstate volume.

F

Fill height

The height of the water surface above the inside floor, usually an inch or two below the rim.

Flat depth

On a bow front, the front-to-back distance at the corners where the curve begins.

Footprint

The shape and area of the tank floor, seen from above.

I

Imperial gallon

The UK gallon, 4.546 litres. About 20 percent larger than a US gallon.

Inside dimensions

Measurements between the inner faces of the glass. The correct basis for volume.

L

Litre

One cubic decimetre, 1,000 cubic centimetres. 3.785 litres make a US gallon.

N

Nominal volume

The rounded gallon rating a tank is sold under. Rarely equal to the water it holds.

P

Porosity

The fraction of a substrate bed that is open space between grains. Around 0.38 for gravel, lower for sand, higher for soil.

R

Radius

Half the diameter. Volume formulas for round shapes use the radius squared, so measuring it accurately matters.

Rimless

A tank with no plastic frame, usually built from thicker glass.

S

Specific gravity

Density relative to pure water. Reef water at 1.025 weighs 2.5 percent more than fresh water.

Substrate

Gravel, sand or soil on the tank floor.

Sump

A second tank, usually below the display, that holds filtration equipment and adds to system volume.

T

Total system volume

Display tank plus sump plus refugium plus plumbing: all the water that shares the same chemistry.

U

US gallon

231 cubic inches or 3.785 litres. The standard gallon in North American aquarium sizes.

W

Water weight

About 8.34 pounds per US gallon of fresh water, 8.55 for reef water at specific gravity 1.025.