If Colin is wrong and it is indeed Columnaris, the fish needs the right type of antibiotic to cure it. Columnaris comes in a variety of strains. Some are so virulent they kill fish before any external symptoms are visible. Other strains towards the milder end of the spectrum can hang around for a long time but if left untreated can still result in death.
There are a few clues to if one might have columnaris or something else. These cues are in the common names we see for Columnaris. One is "mouth fungus" because it appears on the moth of the fish and appears to be fungus but is not, it is Columnaris. Another name is "saddle back disease" because a white patch will form at the base of the dorsal fin.
Here is a decent article which may help. However, they do get one thing very wrong and they do not even seem to realize they they tell you why their suggestion to use an H tank is wrong. They rightly point out that Columnaris is highly infectious which means other fish can catch it from an infected one. Therefore, I strongly advise against using a hospital tank to treat it if you know you have this disease. You must treat the whole tank to prevent it from progressing to other fish. We have no way to know how exposed the other tank inhabitant may be if we know one has this disease.
https://fishlab.com/columnaris/
Columnaris also affects the gills of fish and is hard to see there. I have had decent luck treating Columnaris over the years. I have lost very few fish from this disease once I was able to diagnose it. I have always treated an entire tank which even included a 75 gallon. That is a lot of water to treat and is not cheap. But I had close to 40 imported redline barbs in the tank being quarantined. I had to treat them all.
All of this said, Colin is absolutely correct that you need to be sure you have Columnaris before you decide how to treat. Better pictures would certainly help. Most fish medications are stressful for fish and stress is one of the major reasons fish come down with the things we will need to treat. But, there is also a balance between the stress drugs can cause v.s. the benefits they can provide by effecting a cure.
Unless one has no choice and is forced to take their best shot in terms of a treatment or else fish are dying, then it is normally better to know what we are treating rather than just throwing meds at a problem. When it comes to treating Columnaris, I have read papers where they are having some success using Choramine-T to treat this disease.