Hole Saw Options
For plastic tanks, any holes saw sized 50-65mm will work fine.
For concrete tanks, a diamond core drill bit will be the best option. We use this one available online:
A cheaper option is this one from Bunnings, but if the tank wall is thicker than 25mm you'll need a cold chisel to chip the center out half way through:
Installing the Waterwatch T35 on a concrete tank: