With any foundation design, the concrete comes second to ground conditions. Foundations for aqueous retaining/holding vessels/structures need to be analyzed for both the fully loaded and empty states as a foundation designed only for the fully loaded state can experience uplift when all load is removed and in extreme cases, failure. There is also dynamic loading to consider for when the tank is filling or emptying.
You will need to consult with an Engineer to ensure all proper design criteria are met. I would recommend liaising with your S.E. whom will then get in touch with any other relevant branch of engineering.
Good luck with your project.