Abstract: A removably insertable internal containment reservoir for a storage tank includes a container having an outside surface and an inside surface and formed with a chamber. The chamber has an open distal end and a closed proximal end. The closed proximal end is formed with one or more holes for providing access to the interior of the storage tank. A collar is peripherally attached to the outside surface of the container. The collar is engageable with a flange installed on a manhole assembly of a storage tank. The collar is located a known distance below the distal end of the chamber to position the proximal end of the chamber a second distance within the storage tank. Means are provided for closing the open distal end of the container. Means also are provided for installing in one or more of the holes formed in the closed proximal end of the container a variety of devices depending on what is stored in the storage tank.