In the US, a fish tank's capacity is measured in gallons.
the fish tank
The official SI unit for volume is the cubic meter, but for a fish tank, I would use liters. One liter is one cubic decimeter (or 1/1000 of a cubic meter).
Litres.
It can have any number of measurements and shapes, and still contain its full capacity of 30 gallons. 36"W x 12"D x 16"H is a very common basic rectangular size
Obviously the answer depends on the capacity of the tank truck and this capacity is not fixed by any standard. If each truck carried 4000 gallons, it would take 2000 truck loads to haul 8 million gallons.
To calculate capacity, you need the length, height, and width of your tank
the fish tank
fish tank
Fish tanks are measured primarily by their capacity. In the U.S. they use gallons as the main unit. A fish tank can be anywhere from 1 gallon in capacity to 200+ gallons.
litres
Litres. (L)
Measure the tank. You can get fancy and subtract the thickness of the walls (and the gravel on the bottom).
metric unit
centimeters are a small unit of measurement. Pools are very large. You would use it on a fish tank.
Degrees Centigrade.
km
Gallons