I can't see a good reason not to use the tank, as long as the water line is well below the crack. Since there will be about 40% of the height of water in the tank as it is designed to hold, it should be plenty strong enough. But, how will you make the crack invisible? Since a riparium is intended, as I see it, to be a beautiful arrangement of plants more than anything else, a crack could distract attention from the plants. I know we are told that keeping the glass absolutely clean is half of the battle.