Fittings are the connectors between tubes/pipes and the various components within the water-cooling loop. There is a wide range of sizes, shapes, orientations, and angles for fittings. The type of fittings required would depend on the type of tubing you choose and what angle is needed to make the loop simpler to complete. The benefits of different fittings are to simplify bends from causing difficulties and create better clearance between components.
Each component within the loop will need two fittings, one for input and one for output.
Fittings are mostly a universal size called G 1/4 to connect to the water blocks. The pipe sizes have variants, however Bykski us 14/10mm OD/ID (Outer Diameter, Inner Diameter) since this has become an industry standard.
All fittings have some sort of sealing technique, our standard hardline fittings have 3 inner sealing O-rings with one outer sealing O-ring combine with a compression collar. The Flex fittings are quite secure and simple to use with a barb and compression collar you create airtight seals that last years. Therefore Flex tubing builds are recommended for first time builders.
Coolant is the liquid within your cooling loop responsible for transferring the heat.
PC coolant is specially formulated to minimize corrosion and bacterial growth as well as being thermally resistant. Coolants also come in many different colours and pre-mixed bottles that simplify the process.
Using distilled water would work however distilled water does cause corrosion in time and growth of algae. If you are willing to do routine maintenance with anti-bacterial kits this could be an option.
Coolants are also specially made to be non-conductive, but over time roughly 2 weeks they start to become conductive because of the constant contact with the metal in the water blocks.