6mm glass isnt very strong. I have a few big tanks and have personally seen larger fish destroy tanks simply by swimming fast and turning suddenly so the water wave justs pops the side right off! The fish doesnt even need to touch the glass to destroy tanks. On the other hand, I have also had larger fish smash into tanks so hard I instinctively wince and can actually feel the impact clear accross my larger fishrooms (9mx21m shed).
Obviously the larger the glass and the deeper the tank, the easier it is to break. To me, the fish is more valuable than a glass tank so why take a risk? Anyway to answer your question, Pacu ARE tankbusters.