Other way round - the tubing goes over the rigid Flow tubes. The ID matters, the OD does not. The ID you want is 1" inside. 
I think Michael may have been referring to the OD of the Flow rigid tubes, which is indeed approx 1" or just over 25 millimeters. Still consistent with what I said above - for the flexible PVC tube, you need 1" or 25 mm ID for the part that you are adding to the Flow tube.