A magnetically coupled, implantable pump system comprises an external controller and an implantable device. A non-implantable magnet in an external controller produces a magnetic field that couples the non-implantable magnet to a magnet in the implantable device. A non-implantable motor moves the non-implantable magnet in a rotational direction. When the non-implantable magnet moves in the rotational direction, a piston coupled to the magnet moves within the implantable device. As the piston moves, an amount of fluid from a reservoir in the implantable device moves out of the reservoir and into an inflatable portion of a gastric band. The implantable device may also move fluid out of the inflatable portion of the gastric band.