Need Help Identifying Fish Disease

I hope this message finds you well. I'm currently facing some challenges with my aquarium, and I was hoping to get some advice.

After a recent 5-day treatment with Jungle Fungus Clear and Kanaplex for suspected columnaris in my tank, I've noticed that one of my Neon Tetras is exhibiting concerning symptoms. There are a few faded spots on its body that appear to have a cottony texture. Unfortunately, the majority of fish that displayed symptoms of columnaris did not survive. All other fish that are still alive have successfully recovered from columnaris (cottonmouth).

I've attached a photo of the affected Neon Tetra for reference. Given the recent treatment for columnaris, I'm unsure whether this is a residual effect, a different disease, or perhaps even Neon Tetra Disease.

Any insights or suggestions on what might be affecting my Neon Tetra would be incredibly helpful.

Thank you in advance for your help.

I am not sure from your pics that what you have is fungus. I cannot tell what if might be if it is not fungus. However, if it is I would treat it with Fritz Maracyn Oxy. I have used this since before Fritz bought Mardel and it was called MarOxy. This is a pretty benign med so if you are wrong about it being fungus the med isn't going to cause harm to the tank. The worst would be that if it is not fungus, and a better diagnosis requiring a different med will have been delayed.

https://fritzaquatics.com/products/mardel-maracyn-oxy

One observation. Columnaris has several common names of which cotton mouth is one. Another is saddleback disease as it produces a white patch at the base of the dorsal fin. It looked to me as if a fish in your pics might have that. I am not certain from the pics. So. you need to post better pics or else because you are there and sing the fish in person, make the decision yourself. The fact that some of the fish recovered would indicate the med used did the trick and those white patches are not still columnaris.

I learned to use the above med when I first started to take an interest in breeding. I was working with corys then and I knew a gent who was a pro at it. He advised me when I pulled cory eggs to add Methylene Blue to combat fungus.But he also warned me that when I saw wigglers I need to get the Meth. Blue out of the water as it could damage developing gills. That was when I decided to get the MarOxy and use it instead. I have Meth. Blue for other reasons but not for use with new eggs.

Incidentally, Meth. Blue is one of the very few things that can counteract nitrite one it is insde a fish. M. B. is a good oxygenater among other things. But is is also a great stainer of things in a tank.

Thank you for the recommendation. I'll consider Fritz Maracyn Oxy and provide clearer photos of the Neon Tetra promptly for a more accurate diagnosis.

One quick question--> "In fish, the operculum is a bony flap covering the gills. It moves back and forth to move water over the gills, allowing the fish to breathe, and also serves as protection against these sensitive parts of the fish."

Do you mnotive if any of the fish have their gill flap open most of all of the time? If so that can be a sign it has gill flukes. I ask because in the pics t=I see some red near the opening of the flap. However, there is some variation in neons and I have seen this is some pics while in others I do not. I am a cardinal and eummy nose fan and have never kept neons. I keep kerris now aka purple emperor. If you want a mouthful, try saying their Latin name once, Inpaichthys kerri.......

in the first set of pictures from Sunday, the fish appears to be covered in excess mucous, which is produced by the fish when something in the water is stressing it out (medication, poor water quality, external parasites).

In the second set of pictures (Monday) the fish looks fine apart from a few scales missing on the back.

I would do a 75% water change and gravel clean the substrate every day for a week and see how they go.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.