9mm or in-line taps are a very useful accessory. They can be inserted in-line just after the connection to the 30 or 47 L plastic reservoirs. This allows you to turn off the water supply if you need to undertake any maintenance. They can also be added to the end of a layout so that now and again you can open the tap and allow the lines to be flushed, removing a sediment or lime scale build-up that might occur over time. This will only take a couple of minutes and only needs to be done every couple of weeks.