Composite tank apparatus and methods of making and using same. An array of flat-sided co-dependent cells with relatively thin skins made of metallic or non-metallic materials. Each cell has at least two flat wall panel portions connected by radiused sections arranged such that all of the flat wall panel portions are either in flush contact with each other or in flush contact with flat supporting panels of an enclosing structure or thermal insulating material lining an interior thereof. The cells are sealed with each other around a perimeter of at least one pair of matching lightening through-wall holes provided in adjacent pairs of the wall panel portions of the cells to resist relative movement and prevent leakage of fluid therefrom. The composite tank apparatus may include an enclosing structure, and optionally, one or more layers of thermally insulating material lining the interior surface or the exterior surface of the enclosing structure.