in regards to securing rockwork. I had a friend in hong kong that kept a 120 gallon tank. his aquascaping had one huge cave in the middle where all his fish hung out at night after lights out to sleep.
however, he had a nocturnal fish that was clumsy that bumped into a supporting rock near the top. it shook up the entire aquascape and the cave caved in, crushing almost all of his fish.
oh, and his octopus got spooked and inked the tank too, he was not happy -.-
even with this story in mind, i dont secure my rockwork, but i dont keep anything big enough to upset anything, except maybe my turbo snails