Water cooling tubes connect every component in the loop with fittings, the size (OD & ID) of the tube must match the size of your fittings.

There are two kinds of tubing options.

Hard Tube

Hard line rigid tubing is made from PETG, Acrylic(plexiglass) ,Glass Copper or Carbon. Hard tubing is aimed at more seasoned water-cooling enthusiasts because of the effort and special tools needed to bend the tubing.
To bend Hardline tubing, you will need a heat gun and a silicone insert to prevent the tube from kinking in on itself. The Tubes are carefully heated to a pliable temperature and bent either by hand or a nifty bending tool is used to assist. Hardline tubes also need to be chamfered on the edges so that the rubber O-rings don’t get damaged by a sharp edge causing leaks at the fittings.

Soft Tube

Soft line flex tubing is made from silicone, rubber, neoprene or PVC. Soft tubing is better suited for beginners for its simplicity to bend on its own and to be easily cut with no special tools. Soft tubing however could be over bent and cause kinks in the tubing which will restrict flow and build up pressure that will force the tubing off of a fitting resulting in a leak.