They will not only knock things over, but also graze on corals that happen to be in their path. They are stronger than you can imagine and pry rocks up to graze underneath. I moved mine into my refugium last year because I was sick of it mowing across my plating montis. It is now about 16" top to bottom and it's two years old. They get huge. Also, if there is not enough algae for them to graze they may sample other things to survive. They eat coraline, which is good or bad depending on your thoughts on it. Very cool animals, but know what you're getting into. A small baby behaves nothing like a large one.