You don't need an RO unit for rays. Waste of money IMO. Im pretty sure purigen works, even with rays, but I just don't see the point in using it. If everything's stable and on track, delaying a water change by a day isnt much of a problem, especially when you're doing enough water changes to match the ray's bio load. K.I.S.S. is the key, the more things you add to your tank, the more complicated it becomes, meaning the more things you have to screw up.