A Uniseal is a firm but flexible rubber-like plastic gasket. When fitten properly, it creates a good, tight seal around a pipe that penetrates a tank wall.
See table below for suitable size of Uniseal, pipe and hole saw to suit your purposes.
As Uniseals are designed to fit tightly around the pipe in the hole, they can be challenging to install. To make it easier, cut some short slots into the end of the pipe, so you can make the leading end of the pipe a bit narrower (see picture).
Also, slather the pipe with some kind of water-soluble, food-safe lubricant, like dishwashing liquid. This helps the pipe slide through the Uniseal. If fitted correctly, you should have no leaks around the pipe.