Cut in the light blue edges and drag them outwards a little to give the tank a bit og a bulge on the sides. Select the polys shown and extrude them by about 0.45 or so. Throw a symmetry modifier on it to make the other half.
Now all you need is a cap for the tank. Create a cylinder like the one in the picture and convert it to a poly object.
Bulge the top of the cap by pulling the rings of edges upwards to make a nice curved bulge. Scale down the lower section of polys and chamfer any edges that need to be sharper. Position the cap on top of the tank.
The finished tank should now look like this:
The tank is held down by a couple of belts so we are going to make them now. The first thing to make is the loop that the belts are hooked up to. This is relatively easy to make. All you have to do is create a box and do some extrusions from the green lines as shown by the red arrows. You can use the symmetry modifier to make the other half of it. Chamfer the edges shown in light blue.
Next step is to make the belt that goes through the belt loop. Create a small box with 5 length segs, 3 width segs and 2 height segs and position it under the belt loop as shown.
Now all you have to do is a series of extrudes to create ta belt around the tank we just finished. One end goes over the tank and the other end goes through the loop and back up. The smaller picture shows how the belt fits around the tank.
We now have to make the clip that attaches the belt on the other side. Create a box with 3 x 3 x 3 segs and extrude the polys shown.
Select the top poly and inset it as shown by about 1.5 or so.