Do you mean the tubing inside the Stenner head, that gets compressed by the rollers? That stuff is special, a highly flexible and chemically inert material. The "official" stuff is called Santoprene, a type of silicone I think, and is sized to match the head, and includes compression fittings. Diameter too small and the rollers will pull it through, too large and may overheat the motor. The other tubing attached on both ends of the Santoprene by the compression fittings can be the quarter-inch black polypropylene that is UV resistant.
Be careful -- tubing not designed for a peristaltic pump will quickly fail and you'll get your chemical running out.