Look at BRS video page they have a excellent video on how to install a add on membrane on there kits at least VERY good at explaining. Also they have a video on the flow restrictors. On my BRS 75g unit I didn't have to change it when I added the 2nd 75g unit but I think the one they use on their 75g unit is one that has a wide range of flow but check it out thye do a good job on explaining what restriction # do what GPH I think the 550 is what comes on the BRS.