Fluid Tank
Fluid tanks are Fluid System components used to store liquids and gases.
The maximum amount of fluid stored in each is:
Small tank: 31.25 L
Medium tank: 187.50 L
Large tank: 703.125 L
Density
Gases do not have any mass.
Liquids stored in a fluid tank weigh 0.1 SWMU per litre. Liquids stored in custom tanks instead weigh twice as much (0.2 SWMU per litre).
The official reasoning for this behaviour is to make ballast tanks more impactful.
Gravity
Due to a maths error, gravity is incorrectly applied to liquids. Liquids do not add inertia to a vehicle, but gravity is calculated regardless. This leads to vehicles falling faster than they should when carrying large amounts of liquid.
This affects all types of fluid tank.
Custom fluid tanks
Any completely sealed room can be used to store fluids. A liquid meter is not required.
Note: Custom tanks can fail to spawn correctly due to the Drag glitch.