Something must be wrong in your setup. CNT is one-dimensional system, and it can be modeled by a supercell in the DFT calculation. If in your calculation the periodicity of CNT is chosen along the z direction, the lattice constant c of supercell will be the periodic length of CNT, and the other two lattice constants of supercell should be larger than the diameter of CNT by 10 Angstrom or even more. In this case, the reasonable setup for the k-point can be chosen as 1 x 1 x n, in which n depends on the periodic length of CNT. For (5,5) CNT, you may try 1 x 1 x 10 or even dense k-mesh (e.g., 1x 1 x 15, 1 x1 x 20,....).
