Ok so I have an Emerald Cory Catfish that has recently become ill. It hides in the foliage all day, and only ever comes out to feed. When it started doing this, I noticed a white area near its upper back. It did NOT look like ich, but I could be wrong. I also noticed that it's fins started to tear. The only times it ever gets in a skirmish however is when my Bristlenose Pleco feels territorial.
I have a 20 gallon tank, and the pH is 7.0. It's been set up for almost 3 years now, and I have not had any ill fish for 6+ months. I change my water every week (about 5 gallons), and my fish are on a healthy diet. None of my fish are aggressive, except occasionally my Pleco.
My Cory Catfish is getting near the end of her lifespan (Will be 3 years old in August), and I want to know whether she is just dying from age or what she is infected with.
Here is what she looks like: Imgur