SpaceX plans to power its Interplanetary Transport System with methane and liquid oxygen, both of which can be made on Mars, and to build most of the rocket and its tanks from advanced carbon fibre. The company showed a full-size development tank for liquid oxygen, which Elon Musk described as the largest carbon-fibre tank ever produced.
In November 2016 SpaceX towed the prototype out to sea on a barge and pressure-tested it, reporting that it hit both of its pressure targets, with full cryogenic testing to follow.
via SpaceX Tests Huge Fuel Tank for Mars Colony Spaceship | Space.com