I have a bunch of rock that I found locally, it will be going into a african cichlid tank, and a brackish tank so I am not concerned about it buffering the hardness up i could actually use that. My question is concerning rocks and sand. I have heard that when you put rocks into an aquarium that you need to put something under them other then sand like rubber, because the point loading of a rock on a few grains of sand would be enough to crack glass. I am a mechanical engineer and this seems like a very realistic event, so my question is what could I use for padding under the rock that would be chemically inert and fairly easy to obtain, I am in the upper peninsula of michigan and stuff is hard to come by up here.