Thank you very much for the feedback!
The elements show how much input and output they have (each container has a little ‘window’ where you can see the liquid level), but I understand that it is very subtle. I will add a popup with this and more informationwhen hovering (or clicking) over the element.
Regarding the enemies/fuel issue, it is briefly explained in the tutorial. I'll explain this in a more visual way, at a tutorial level, where you can see the mechanics clearly.