I have 5x water butts plumbed in series, they are filled from the garage rainfall guttering, I made an adaptor that allows a normal 36mm pipe to be connected to the feed system aswell as having a feed from the black corrugated pipe, the adaptor was printed in white ASA (which is UV resistant & doesn’t go brittle) as it gets a lot of UV rays on it. I also made these guttering adaptors to accept a 36mm pipe aswell as the guttering which was installed on the garage.