I would actually glue them down with marine epoxy. With some fish and even some inverts will move a pretty nice sized piece of rock and will cause a huge crash. It's a suggestion from what I see in your picture on rock placement. Talking about marine epoxy, I need to get some myself... Having problem making my bubble coral stay in place because of the darn hermit....The Hermit stays even though he causes crashes of my corals...He does a GREAT job going to places where my other stuffs wouldn't go to clean....Especially by the Long Tentacle Plate Coral, NOTHING goes near by it except my fishes, they tested before hand and learned that it can't harm them and so did the two exotic hermits, my other small blue legs and scarlet won't go near by it. :happyfish