Hey Hiph-Op, welcome to Pixelation
This is a fun design. My first suggestion to you is going to be the same as my first suggestion to a lot of people, which is to start thinking about your art as 3d volumes that you are rendering, rather than an outlined coloring book picture that you are coloring in. This isn't to say that lineart is bad, but its clear from the way your image is shaded that it was lineart first and foremost, and an actual object in space second. A good exercise for this sort of thing is to do a quick sketch of this object using just 3 shades of gray, instead of outlines. Don't spend more than maybe 15 minutes on it, but when you are finished compare the results to what you've drawn already, and use that to add more authentic/believable lighting to your tank.
