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 or liters
The best unit of measure to use when measuring the amount of water in a drinking glass is ounces. A liquid measuring cup can be used to measure the ounces of water.
millilitres
take a cylindrical measuring thing and measure the amount of water in it then drop the item in then measure that amount and subtract the first amount from the second
Use water displacement. Measure water before you put the rock in it, and then again afterwards. Put a measuring cup large enough to hold the rock in a pot large enough to hold the measuring cup and fill the measuring cup to the very top with water. Lower the rock into the measuring cup carefully so that the water flows over and into the pot. Carefully remove the measuring cup with the rock without spilling any additional water into the pot. When you measure the water in the pot it will equal to the volume of the rock (accuracy will depend on how careful you were). This is displacement measuring.
by measuring the amount of water it displaces
Measuring cup
You have to use a measuring cylinder and water.
I woould use a small measuring spoon, as in the image
It is a measure of volume usually used when measuring the water in a lake or large reservoir. An acre foot is the amount of water that it would take to cover one acre of land with one foot of water.
If you want to measure it's volume, place it in a measuring cylinder containing a certain amount of water, and measure the extra units to get the volume of the object
If you have a 1 ml measuring spoon.
Throw them all into a suitably sized measuring vessel marked with volumes and measure the amount of water they displace.