If you have purchased or are using your own large tank and do not have access to insert a 16mm water barrel tap you can use a 16mm Top Hat Grommet.
A 19mm hole is drilled at the bottom of the tank and the 16mm Top Hat Grommet is inserted into the hole. 16mm pipe (hosepipe) is then pushed into the grommet. This will form a water tight connection
6mm Top Hat Grommet