A fluid tank is a 2x2 machine used to increase a Spaceship's shield capacity by 500. Without fluid tanks, generators can not be used and the ship's maximum shield capacity is 2000.
Fluid Tanks needs to be directly connected to a Shield projector to increase Spaceship health. They can be connected to additional fluid tanks to increase capacity and shield generators to increase regeneration speed. Fluid tanks leaks fluids overtime and is increased by other fluid tanks connected to it.
The Fluid Tank can be crafted at a Machine Fabricator using 64x Iron. As with most items, they can be acquired via trading or fighting.