Columbian Shark just received not eating

[Becauseweare]

Hi, I am seeking for help, I just have my first columbian shark and he doesn´t want to eat, I place him on a 65 gallons tank for now, eventually move him to my 210 gallons tank, but he keeps swimming around the surface of the tank and he stopped eating, he is hungry because he keeps eating a piece of floating plant and try to nibble the pellets but he lost interest that fast too, please help?

 

[Circus]

You have it in brackish water, right? Lots of flow or an airstone?

 

[Colin_T]

Check the water quality for ammonia, nitrite and nitrate.

Increase aeration/ surface turbulence to maximise oxygen levels in the water.

Post pictures of the fish so we can check it for disease.

What sort of food are you offering it?
What was it being fed before you got it?
What was the pH and GH of the water at the shop, and is it similar to yours?