I have seen many pictures of areas where darts are collected with hundred foot high waterfalls. I am sure a dartfrog knows somewhat not to get underneath it. I cant think of a tank that would be tall enough in my opinion to cause that kind of a threat. I have a waterfall with a 15 inch drop that has a lot of water going over it, 300 gph. I have seen my mantella viridis under it many times, without any injuries.