What is the meaning of having a whale in a tank dream?

This dream brings to mind the saying "a big fish in a small pond." That means that a person may seem to be important or special when the reference group is limited, but when compared to others in the larger world, that person is not very special or important at all. The dream could share the meaning of that expression. Alternatively, the dream might suggest that the dreamer has a problem that will soon outgrow the current situation.