My black moor get sucking in my aquarium siphon durning water change. Is there any medicine or treatment to follow to straighten the fish's back/spine?
Similar symptoms might have very different causes. And the different causes lead to different treatments and different outcomes.
In your case, the best thing you can do is to allow the fish to live his life in the main aquarium - and prevent it from being sucked or harmed again :)
In my case, my Betta had a bent spine. I assume that it happened because the water temperature was quite low for some days - and he might have caught a cold or something. After the water temperature raised to normal levels, he slowly got his straight spine back.
Joke: both our fishes might have a benefit from some therapeutic massage, but it is unlikely that either of us will go to that length :)