Gallons.
I am guessing you can use Gallons.
In metric system it is 'litres' or 'kilograms/tonnes'. In the Imperial system it is 'Gallons' or 'Tons'.
you dont count you measure it
Measure its dimensions in inches or centimeters, and its volume or capacity in gallons or litres.
the amount of water
In a rectangular aquarium, multiply the length, width, and height in inches and then divide the answer by 231 to get the amount of gallons the tank can hold.
They're measured in gallons in the US, and liters everywhere else. This holds true no matter how big the tank is--the New England Aquarium in Boston has a tank they describe as holding one million gallons of water.
gallons
Litres for a domestic aquarium, kilolitres for a large [public] aquarium.
Kilograms (kg) measures the amount of matter in an object. (Mass)
gallons
If you are trying to measure concentration of salt in water you would be measuring molarity which is moles / liter. Moles are equal to grams of salt / the MW of salt (and if you mean NaCl then the MW is 58.44 g/mol). Density is a ratio of mass to volume (mass / volume or kg/m^3)