Hi,
Fluid80 element is legacy type contained fluid element, but for sloshing it should have a free surface, ie. not fully contained. You can use the current technology element fluid221 or 220 instead. It is recommended to have share topology between your fluid body and the tank, ie. shared nodes between fluid and structure. Or at least define MPC contact between them if share topology creates difficulties for meshing (which shouldn't be an issue for your model as it is not a complex shape). Fluid element (uncoupled) doesn't have displacement DOFs so I don't think the coupling command is going to have any effect on it.
What kind of results are you interested in seeing?
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