I had the same thing, my lumphead cichlids were living in a tank with big gravel for a few weeks while their new tank was being organised, then I moved them out, and raised the salinity in the old tank to 1.010, and added my 2 puffer fish. About 3 months later, when the salinity was about 1.018 (almost marine), I found about 10 fry, who were big and happy and healthy. They're still alive now (breeding themselves, actually!) 6 months on.