Curious... Are there any rules on what type of rock you can use in a planted aquarium for hardscaping? I know that some type of rock contains mineral or metal traces that could be bad. I am thinking of using some smaller flagstone or slate pieces and just want to make sure it would be safe for plants and fish. Also, any ideas on what to use to hold this together in a structured wall piece? I have heard silicone and even read of a guy that used mortar. Any input, ideas, or sources of information would be greatly appreciated here. Thanks!