if you put a rock in water you would find that the rock is not a whole lot more dense than water.
so the force that the rock exerts both downwards and sideways is not that much more than water.
so as long as the main vector force is downwards and the leaning force sideways on the back glass is just slight the tank shouldnt be exposed to a lot more stress than it would experience by just holding only water.
and in fact if a rock falls in water ...it is a much softer decent than in air like reduced gravity.
so the chances of it cracking the glass are much reduced.
thats why you should do all your rockwork in a full tank of water.
so the force that the rock exerts both downwards and sideways is not that much more than water.
so as long as the main vector force is downwards and the leaning force sideways on the back glass is just slight the tank shouldnt be exposed to a lot more stress than it would experience by just holding only water.
and in fact if a rock falls in water ...it is a much softer decent than in air like reduced gravity.
so the chances of it cracking the glass are much reduced.
thats why you should do all your rockwork in a full tank of water.