It's nice and easy to figure out the weight of just the water. 1L equates to almost exactly 1kg, thus a 300L tank, the water would weigh 300kg. Obviously when you add substrate this will weight heavier than water, and any ornaments add extra weight - not forgetting the actual tank itsself.
As others have said before me, it depends on the actual glass the tank is made from, thicker glass is obviously going to weigh more than thinner stuff.
Use it as a rule of thumb - if you can estimate the weight of the tank empty by picking it up, add the weight of the water and about 25kg for substrate/ornaments, and you won't be far wrong.