It's simple provide a path that is a simple half circle at the bottom and then use your existing path as the guide curve. (See attached.)
Also, Your side fill surface WILL NOT be tangent with your swept surface (use deviation analysis to see this), you'll see that I changed your Sketch8 into an extruded surface so that I could then derive tangency from that boundary when doing the side fill surface. Instead of the sweep, you might want to try two half circles, existing at bottom and one at top and use Boundary surface instead - make sure to turn on the "next curve" option in the direction of your side profiles.
tank 21-mab.SLDPRT.zip 299.2 KB
You sir, are a genius! Thank you!