How does the storage tank factor into your model? Are you looking at volume over time? Does it store so much water that it can offset water shortages? Are there water shortages in groundwater?
Not all parts of a system's infrastructure need to be included in the WEAP model. If the storage tank is there as a place that the water passes through, but it doesn't change the way water is supplied, you probably do not need to include it in your model.
The question to ask is what drives the way that water is supplied to your domestic site? Is it the groundwater supply, or does this storage tank play a role too?
Either way, I would not use a demand site as the storage tank, because it has parameters to drive demand, not pass water through.