Fin Rot

My fiance' and I have a Pangasius (Iridescent shark) who is several years old now, probably around 4-5. Anyway,

He recently developed what seems to be fin rot. BUT, he's also got it on his sides, and it's turning into this little white junk on his sides. Looks like he's molting, or like fluff coming out of a stuffed animal??

At the same time, the tank water clouded, and clouded bad. We've changed the water (75% change) and are treating the fish for the fin rot. We also have a blue gourami (spelling???) in the tank who is about as old, and is fine.

Does anyone have any suggestions or thoughts on what to do. The medication we're using is Maracyn.

Well, it seems to be clearing up. I believe the Maracyn tablet caused the water to cloud up, but it's working. The molting looking junk on his side is clearing up and his appetite has gone back up. The fins are still red at the base and a little tattered, but that takes time. Thanks for the help everyone.