If the lid has gotten wedged on in shipping, it may be very hard to remove initially but still operate normally once you get it loose. If you have one, you might try one of those jar lid openers, a textured rubber sheet that lets you grip an object with more friction than your skin will provide; use that to hold the lid, while holding the tank body in your hand, and twist while lifting the lid away from the tank. Failing that, filling the tank with warm to hot soapy water and then drying the exterior (to restore gripping ability) might expand and lubricate things enough to get the top off. Do rinse thoroughly afterward, of course...
Note that heating the whole tank will slightly loosen the lid, because the lid, a slightly larger diameter, will expand more than the tank at the same temperature. Heating only the lid would be better, but is very hard to manage when the parts are tightly joined.
If all else fails, a scrap of wood driven with a light hammer can be used to drive up the edge of the lid, working around to even out the movement (though this option really requires 3-4 hands). Using a wood scrap instead of, say, a screwdriver will reduce the chance of bending the lid's rim and permanently damaging the tank.
