The tank was the largest item ever moved by astronauts during a spacewalk.
The planned 13-day mission is designed to transport new equipment and experiments to the space station to boost its capacity as an orbiting laboratory.
The crew of six US and a Swedish astronaut brought two 'racks' for scientific experiments and a freezer to store experiments set to be sent back to Earth. One of the racks will be used to conduct research on metals, semiconductors, crystal, glass and other materials in microgravity. The other will be used to study liquids and gels in space.