In the above screenshot it shows the GUI from the Polymerizer with full natural gas down the right side and bottom and I left the tank.png without a transparent background to see where the overlay is. This makes is difficult to re-texture the GUIs.
If you look at an untextured tank in the machine GUI you can see the tank.png is 3/4s overlayed on top.
The Tank in the GUI seems to be split between 3/4s tank.png and the rest from the machine/block GUI, this is highlighted when re-texturing.
http://postimg.org/image/420d9r4cp/
In the above screenshot it shows the GUI from the Polymerizer with full natural gas down the right side and bottom and I left the tank.png without a transparent background to see where the overlay is. This makes is difficult to re-texture the GUIs.
If you look at an untextured tank in the machine GUI you can see the tank.png is 3/4s overlayed on top.
http://postimg.org/image/fsear4x55/
I've pointed to the places where the GUIs are overlaying
http://postimg.org/image/fgwuedgp5/
The issue is tank.png is not drawing complete when there's fluid in the tanks.