This is the 11657th most frequent Russian word.
жидкостью
The Russian word "жидкостью" means "liquid" or "fluid" (instrumental case).
In this sentence, 'жидкостью' is the instrumental case of the noun 'жидкость' (liquid), indicating the means or the material used for an action, specifically what the tank was filled with.
Он наполнил бак жидкостью.
He filled the tank with a liquid.
Here, 'жидкостью' refers to the quality of flowability of the paint, used as an instrumental case to describe one of its properties.
Эта краска обладает хорошей жидкостью.
This paint possesses good flowability.
In this context, 'жидкостью' is again in the instrumental case and refers to the means or alternative substance used in the experiment.