From YT:
LFilippone They don’t live well in tanks, if you put them in a in ground pond they will grow 2x faster healthier and larger, since they are not used to seeing predators on the side, they dart away in tanks, in a pond, they will simply dart to the deepest part and sit. Your tanks are some of the healthiest I’ve ever seen, and it is still not doing well, I think this is a fish that just isn’t cut out for captivity
Fish-Story Totally agreeable on all points, thank you, very helpful! Except I'd doubt the predators on the side argument. That's where all predators of water appear usually. I'd think it is mostly that they are spooked by being able to see a change of light, shadows, and movement of something very large (a human figure) behind the transparent wall, so the wall better be opaque, I guess, as in a pond.