The one thing I have learnt about Thingiverse is that by my guess some of the people who design items on there have never 3D printed anything in their lives.
Depending on the format of the downloaded files there is a way to get thicker walls. It might be possible to open the file in Openscad or Blender, turn it into a solid lump, not hollow, then create another one but scale it down by say 10%, and deduct that from the original. I do it all the time in Openscad as it is easy in there, it allows me the create a complicated part, and then hollow it out with an even wall thickness throughout.
The other problem with downloaded files is that they may have created them with a low polygonal count, so the part is low resolution, curves are rough etc. Again, if it is created in Openscad it is a simple $fn=100, to fix it, though I assume that the same can be done with an STL file in Blender etc.
Your dedication to printing this tank is beyond the call, I would have abandoned it long ago,
