the ph seems a tad low to me...
the rock could tumble down thanks to a single snail, depending on how its stacked and whats under it, like if its sittin on sand, and a nass snail were to burrow under the rock, the sand could shift, and tumbel goes the rock.. the other thought would be if you end up with an eel, like i have, i'm amazed how strong he is, and how much he likes to burrow in the sand between the rocks... again....