I have had a 20G tank for almost two months, but every time I add fish to it in about two weeks or less they die. I have checked the ph levels and the nitrates and all the good stuff I have two dwarf Guaramis two white clouds that have survived this whole ordeal and two plecostomus that are survivors also. I have tried to get what I have read and heard are somewhat hearty fish like platys and mollies but it was unsuccessful. Please help I am lost