Tropical fish tank, one fish is not moving much?

Unfortunatley, fish that you buy from a fish store are often already sick when you get them. That is why recieving tanks are recomended. You have to expect about a 50% fish loss. I treat all my new fish with an antifungal dip before I introduce them into the tank. It increases thier survival rate,but there is only so much you can do. Many fish like to rest on the bottom. Don't be too concerned about that behavior. They will often chase each other around. It may look aggressive, but if there are no visible wounds then they are just establishing a pecking order and playing games. A good rule of thumb is to always buy a few more than you really want and the healthiest will survive in a well kept tank. Good luck .