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
While you are compiling info, comment on what is gong on to minimize the problem. Where do you stand? Working for owner? a contractor doing this? who decided to do it?
What material is the tank made of and how long has it been there? With that much settlement in close proximity to the tank, you might have soil raveling into the tank, assuming the tank bottom is below the footing.
As OG noted, a cross section with dimensions would be helpful.