So one of my goldfish, Fredrico is experiencing terrible lethargy and looks like he might have fin rot. It’s sort of hard to tell as he’s always had sort of frayed, not-perfect fins, but I’m thinking they’ve gotten a lot worse so I’m treating it like fin rot is the issue. I’m not sure how he developed fin rot as I test my water frequently and have never had ammonia. I have had this fish plus his tank mate for years with no issues. Fred has always been an active fish until now. Currently my parameters are 0 ammonia, 0 nitrite, and under 10 nitrates. His tank mate, Molé, is active and has a beautiful flowing tail with no signs of fin rot. Fred rarely leaves the bottom of the tank for more than a few minutes for the past 12 days, and his fins are pretty frayed. He does have a good appetite, though. I currently have aquarium salt in the tank (1 tbsp per 5 gallons) and I tried treating with a round of API E.M. Euythromycin. I have an order of Fritz Maracyn coming in the mail soon to do a second round (API’s euythromycin product was taken off the market). I am testing my water everyday and have 0 ammonia. I do notice that my guy has a bit of a bulgy side (picture above), and I am not sure if he has always looked like this or if it’s new, but I haven’t really noticed it before. He has been missing a fin on his bulgy side since I took him home from his previous owner, and I think he over compensates for it and has developed a bit of a deformed body shape (he has never been a good swimmer and his missing fin definitely does not help). But I wonder if maybe it’s a tumor? I wish I had taken more pictures so I could go back and look but unfortunately not :(. What do you guys think? What else can I be doing? The thing that is confusing me the most is his tank mate is 100% fine with no signs of illness? What do I do next??? 😩

At a loss on how to help my sick fish