I don't have time to look at your files (post a pic) but the obvious first question is: did you restrain the tank somewhere?
I looked through your study setup and I think I found the source of the large displacement. In the definition of the pressure you have set the pressure to be 9810 N/m^2 but in the Nonuniform Distribution settings the units are set to millimeters. If you set the Nonuniform Distribution units to meters and then rerun the study you will get ~8.8432 mm of displacement instead of ~8.8 km.
Thats great Rob, Thanks very much for taking a look at this for me. I cant believe it was something so simple!