A replaceable tank for an ink cartridge used in a bubble jet printer comprises a container formed by side walls, top and bottom walls connected to the side walls. At least one ink supply port is provided in the bottom wall of the container and is disposed for supplying ink from at least one ink chamber in the container into a corresponding ink tapping pipe in the ink cartridge. The container includes a locking member on one of the container side walls for engaging a guide groove in the cartridge, the container being configured for insertion into an open top of an ink tank holder formed by the cartridge, with the bottom wall of the container facing downward and the side walls of the container aligned with side walls of the ink cartridge. The locking member is constructed for securing the container in place in the ink tank holder with the ink supply port connecting with the corresponding ink tapping pipe.