Method 1)
Take a "volume" measuring device, and fill it with the total amount of water, the bottle can hold.
We then put the same, empty bottle into the container, in a way, that it sinks to the bottom of the akwarium. We can then measure the new height and multiply it by the volume of the liquid.
This will clearly give us the volume of a bottle.
Method 2)
We could pour water into a 0,5 litre coke bottle, and weigh how much it would weigh.
We could do the same thing with an empty bottle. The difference of the two would give
us the weight of the water. …
