curved back

Aquarium set up: 240 Gallon 2 Fx6 Canisters I just performed a water change when I got the below picture Once very eager to eat and now is not How can I treat him and make him overall more healthy?

Hey I need everyone’s help! I got a baby alligator gar 3 days ago and the first day he was fine I woke up the second day and it looks like his back is curved or arched. He is still swimming and fins are still working but doesn’t look like as well as before. I don’t know if he is eating or not...