i did this and it was sooooo much fun, i'm extremely impatient and it took roughly a week to do my rock wall and cave (i was hoping it would take two hours lol) but it turned out wonderfully!! I would use tile grout if i were you.. Worked great for me!! I'm going to build another one make it look more natural, theres many improvements i could do to the tank its such a fun project!! have fun!