How do I make a rock wall?

[heartagramtc]

I am in the process of bulding a corner tank, and I am building a false floor I guess you can say my tank resembles waterkeepers build. So my question is for the front part that hides underneath the ff, what kind of adhesive, can I use to make a "rock wall" so that way I can place rocks and sand to make it look like its a well rock wall. I do plan on having about 3" of water and live water plants and possibly fish. I just don't knw what's ok. Concrete, mortar, epoxy, silicone? Thanks all for your help!

[Green Ghost]

I do it out of concrete but it's very long. I guess there are many ways to do it also. You could even silicone rocks together and sprinkle some sand between.