In my tank, there is only one rock that is glued in place, the grapefruit-sized one on the very top. The red line shows where the glue seam is. The potential for disaster should this rock
dislodge and fall is apparent; my glue seam had to be strong and reliable.
What I did (and do for any gluing operation, be it a frag plug or whatever), is to knead a ball of epoxy and form a "pancake" the appropriate size and thickness to the surfaces to be joined.
The thickness of the "pancake" should be a bit thicker than any gaps between the irregular surfaces to be joined. Your goal is to get as close to full surface adhesion as possible when the rocks are mated.
Apply a bead pattern of superglue gel on one surface of the pancake, enough that when pressed against the rock, the gel will spread to cover most of the surface. Press the gelled surface of the epoxy
into the surface of the first rock, making sure the epoxy fills as many nooks and crannies as possible.
Run another bead os superglue gel on the exposed surface of the epoxy as before. Position the glued rock and press into the intended surface. Gently rotate while pressing the glued rock to ensure the epoxy fills as many nooks and crannies on the receiving surface as possible. If the pancake was made thick enough in the first step, you should be able to get nearly full surface coverage. In a few seconds, you will feel the superglue gel "lock". When you feel this STOP, otherwise you weaken the newly created bond.
The superglue gel will hold the rock in position until the epoxy cures. Any epoxy that oozes from the seam (hopefully, you will have some) will be covered with coralline in due time and will be indistinguishable from the rock.
I glued the rock shown above nearly a year ago and it remains solid. Every so often, while working in the tank, I give the top rock a tug just to make sure. For all practical purposes, it is one solid piece.
The only time I use
only superglue gel is when placing a frag and am unsure of its final position. When I am certain of the placement and want to "lock it down", I use this glued epoxy technique.