Give us a cross section showing footings to scale, the tank and complete soil information to depths at least as deep as three times footing widths (density (SPT),water conditions, etc). Photos would help, if possible. Loads on footings and resulting soil bearing pressure. Where data is not available, at least estimates. When did settlement occur and how related to tank history or other site history. What is the area geologically speaking. Area ground water table situation and history here. Even so this may not be enough