Why is my tetra at the top of the tank?

I have a 16 gallon tank with 3 tetra, 3 barb and 1 catfish s sucker. We got them yesterday and all are doing fine but 1 tetra. He keeps going to the top of the tank. He's not gasping for air but he stays in one spot for the most part. His color is fine, ph levels are fine, filter is running not to harsh, water is clear, they are fed, and water is between 76-80... what could be wrong? i think i have all the bases covered..