Slip fittings will not give you the pressure you need to create a seal. Get a threaded male/female slip. Put a rubber O-ring or a gasket over the threaded male end then-Put the threaded male end through the hole in your container, on the outer side of the container, screw a threaded female coupler on and tighten it down until it's putting lots of pressure on the Oring/gasket. I've heard electrical fittings are better than pvc plumbing fittings as the threads do not taper on e fittings. The Orings they sell for plumbing a faucet work great for this.
Basically you will end up with the gasket/oring on the inner wall of your tank, with a nut on the outside of the tank keeping enough pressure to make a water tight seal. Or look up any "bulk head" as that is basically what I'm explaining how to make.