Standard compression fittings for rigid PETG and acrylic tubing work differently than compression fittings for flexible tube. With rigid tube, an external o-ring is placed on the tube after insertion into the fitting. The cap is then slid down over the o-ring to the fitting and tightened. That compresses the o-ring and forces it to expand, trapping the tubing and sealing it at the same time. With rigid tube the inside of the tubing never touches anything but coolant, so the inside diameter (ID) isn’t needed for sizing. TITAN RIG’s selection of standard rigid compression fittings includes examples from the best brands and in a variety of different colors to match the theme of your water cooled PC.