.Personally I'd glue the rocks together. When you start to clean the tank most likely the fish will be hiding under or in between the rocks. I 'glued' a bunch of slate together to go in with my Oscars and Pacus. I used an aquarium glue called "HoldFast" It was around $12 at Fintastic (probably cheaper at other stores). It came as a (approx) 8 inch cylinder in a green package. Ya just break off a piece, roll it in your fingers, mash it into where you want it then put the other rock against it. Real easy to work with, feels icky but cleans up easily. It's not a runny glue like wood glue, etc, instead it's more of a paste/clay type substance