red blister like spot on catfish tail

[coron671]

Hi! I was hoping someone would be able to help me identify and treat my catfish. I purchased him a little over a week ago. After a few days he started to get white spots on his body, which looked like ich and the petstore reccommended treatment with Clout. At the same time he had a whitish-yellow spot on his tail. After treating the tank with clout the spot turned pinkish red. I returned to the petstore and they said it sounded like a bacterial infection and had me get arythimicin and triple sulfer. I did one treatment two days ago and the spot has gotten larger. I did my second treatment tonight.
Below is a link to 2 photos and any suggestions would be welcomed.

Thanks

http://www.shutterfly.com/osi.jsp?i=67b0de21b342193ba4f5