It sounds to me as if your new fish has a bacterial infection and should be treated with an antibiotic as should the rest of your tank, as it is very contageous.  Never get a new fish and throw him in the tank with your old fish.  You should always quarantine a new fish for 2-4 weeks as they could be harboring bacteria or parasites. 
I would think that gentamycin sulfate would be the best choice.  You can get it in powder form, possibly at the petshop.
You should raise the temp in your tank to 84-86 degrees and treat for 7 days.

gosh, that does suck!  I have never seen healthy fish at petco.
Well, next time pick a fish from a tank where all the fish look healthy!
Or try a different petshop!

Jazmyne: I'm pretty sure that his disease simply came from the shop, it's a very common occurence |: Otherwise, this kind of infection is usually caused by bad water quality or bad nutrition.

Just to be safe for your next fish, in case this one dies, maybe you should empty the tank and start over? Disinfect it with bleach to kill the bacteria Then you'd have to start over with the nitrogen cycle, but that shouldn't take more than a few weeks! How big is your tank? And do you know about the nitrogen cycle?