This question is on behalf of my sister who has tropical fish. She has a 4ft well established tank with relatively few fish in it (approx 10 tetra size fish) but on adding a couple of new fish they seem, in turn, to be developing a hunchbacked or `kinked` body. I`ve looked on the internet but it`s just made me more confused with all the scientific talk. Any ideas as to the cause that can be easily explained?