Why is my new African Sideneck turtle acting strange?

I bought a turtle for my girlfriend from petsmart like 2 days ago. I bought a starter set that came with a 20 gal tank, filter, the UV and basking lamps, a basking area, and pellet food. I put him in and he instantly just has been scratching at the walls to barely put his snout out of water like hes gasping for breath but hes an aquatic turtle? The water turned foggy overnight and he just sits under this bridge-like thing we got for him to bask on. He never basks either. Theres gravel too. About 5 inches of water. And today we bought 12 goldfish thinking he'd eat it cuz thats what the lady at petsmart told me. Any advice on how to make him active or something would be awesome of if this is normal?? Thanks!!