I recently started taking care of an established 55 gallon freshwater aquarium for the first time, and I'm in the process of learning all about it. The aquarium contains 3 loaches (2 salmon colored and one brown), 4 goldfish, and 2 plecostomuses.
One of my loaches (about 3" long and salmon colored) is very sick. He started by hanging out at the surface all the time. I noticed that there was a small spot (like an abrasion of some sort) on the side of his face. I immediately went to a fish store and asked the guy about it. He tested my water, which came back perfect, and he suggested that I use Melafix (melaleuca) to treat for a possible bacterial infection.
I have been using Melafix for the past 2 days, but he only seems to be getting worse. He is now frequently floating upside down, and I have noticed what seems to be some hemorrhaging on his body (small cluster of reddish spots). He is still alive, but looking extremely ill. I am really afraid that I am going to lose him, and I don't know what to do.
I would GREATLY appreciate ANY advice. Please, please help! I am so worried.

Thank you,
Katie