The difference between 1/4" I.D. and 6mm I.D. is negligible, but it has been stated that 1/4" I.D. works but is a touch to big, so I suggested getting the 6mm if it is available to him.As 1" = 25.4mm, 1/4" = (25.4/4) 6.35mm.
Thanks I thought 6mm, will order somePeople in the US who have used 1/4" I.D. hose stated that it was just a little to big. That leads to my best guess of 6 mm I.D. as 7 mm would be bigger than 1/4".