Not a fish doc, but something like this is usually a bacterial infection or a parasite. Unless the fish came from sketchy conditions (outdoor fish pond, for example) parasites are probably less likely than a bacterial infection. How long did it take to develop?
Often, a fish can be injured and the wound, however small, can provide access and a foothold for infection. Fish are especially succeptible to this if it is already stressed.
I'd try to treat him pretty quickly and make sure the water is warm enough and clean.
Were he mine, I'd assume 'bacterial', and treat him with a commercial anti-bacterial preparation like 'Melafix'. I know some people use epsom salts, but I don't recall the correct ratio to use and therefore cannot easily recommend it. I have been using Melafix successfully to treat fish with injuries that seem to be getting infected, with minimal impact on other animals and plants, and I'm sure there are other preparations available that you might look at using.
Thank you so much! I will definitely try that. I just noticed it yesterday. I think it can’t be more than a couple days old because today it looked much different. I will definitely try that thank you.
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