You can convert the solid tank to a sheetmetal tank. Using the the same process mentioned earlier, its just that you will have a more difficult time "ripping". You will need to choose the tangent edges of your radii. I'm assuming a 4 sided tank, so if you choose the two tangent edges on one side, then the two tangent edges on the opposite side you should be able to do this. If the flat pattern will have two rectangles bigger than the other two, as these will have the material that becomes the radii. If you have a radiused bottom as well, apply this technique to those edges too.
Hope this is helpful.