Mass of the water is roughly 60Kg providing it is actually full to the brim, plus the mass of the tank, somewhere in the region of 70-75Kg when full I would have thought.
My 60L tank weighs around 75kg I'd say with equipment, water, gravel, stones etc. When buying a shelf and planning for it I just went with the image of it weighing 80kg. That way I wouldn't have to worry about what I put in etc.