Nanotube growth requires a 1 inch quartz tube. The tube sometimes becomes contaminated. The evidence for contamination is typically unreliable nanotube growth, which become reliable again as soon as the tube is changed. Contamination is not visable, unless you gas flows are very slow and black carbon soot builds up on the wall.
There should always be two spare tubes. We've been ordering the tubes from QSI quartz scientific.
Note: The outer diameter of this tube could stand to be a little bigger.