Help! What's wrong with my betta fish?

[Lyndrel]

I bought a halfmoon double tail betta2 days ago. When I first got him, he was extremely lively. But since I've switched him in his tank, he either lies down flat on the bottom or floats straight up and down in the corner of the tank. Occasionally he'll swim to the top for air. At first I thought maybe he has swimmer's bladder because my brother overfed him. So I fasted him for a little over a day, but he's still the same. I even tried to give him some clean water. I tested the water and it seems fine, conditioned it and everything. He does seem to be breathing a little faster than normal. Sometimes he'll float with his belly slightly towards the top...definitely not dead though cause like I said he'll swim to the top for air. I went to the pet store to ask what was wrong...of course no one could help me... so someone please help me on here! :[