you did Modal analysis which supports only structural bodies, for tank filled with liquid you should be doing Modal Acoustic analysis instead which supports structural and acoustic bodies. Apply Physics region on tank and Acoustic region on Liquid volume, make sure to delete viscosity from liquid material properties.

Make sure to insert Fluid Solid Interface on Liquid volume, so interactions between tank shell and liquid are also considered. I would also recommend modelling tank with surface elements (Shell elements in Ansys) instead of solid 3D; it will save you a lot of trouble with meshing, analysis will be quicker and more accurate.

It should look something like on this picture.

I hope it helps,