Currently, gasses like steam currently fill up a tank like any other liquid would, which is not very realistic. What if they filled tanks up like they fill up tanks in Thermal Expansion and Xycraft? Filling up the entire tank, but just changing the opacity of the texture?

i think it's negative dencity or so

negative density just means it flows upwards. isGaseous checks for gases.

How does one determine something is a gas from the Fluid object?

This should be fixed by 0979a09

But what type of liquid is considered as gas?

Are gasses considered as those liquid which flows up?

@liach No.

Those are two states, something can't be a liquid and gas at the same time...

Fluid is the word you are looking for Liach.