my goldfish needs help, please

Question
I found this website on the internet, and I am hoping you may be able to shed some light onto my fish's newest problem. He is a normal goldfish that I have had for about 2.5 years. He has a 5 gallon tank and is the only fish in it (he is a little spoiled). My poor fish has survived college life for these last few years and seems to be fine other than his mouth. recently, he his mouth appears to be enlarged and doesnt shut all the way. he doesnt appear to be in pain and he is still acting normal, but he does seem to have a little more difficulty eating his fish flakes. He had a little finrot a few months ago, but that cleared up nicely and he hasnt had any issues since. do you have any idea as to what is wrong with my fish and how i can help him? I really appreciate it.

Answer
Hi

might be Mouth Fungus, Columnaris (Chondrococcus, Cytophaga)

Despite its misleading common name, mouth fungus is actually caused by bacteria. Patches of cotton-like patches develop around the mouth, but also on the head, fins, gills, and body. As the disease progresses, open sores develop.

Raise the water temperature. Several different courses of action can be taken including a 30 minute bath in 1 ppm potassium permanganate (10 Mg/L); the addition of Malachite green; or the addition Nifurpirinol. Frequent partial water changes are important.