Do a very big water change and dechlor with normal dosing.
Then do a round of treatment with a broad spectrum anti-biotic. This may damage the bacteria in the filter. Mardel Maracyn 2 would be an example. You dose it for 5 days. It can be repeated for another 5 if needed.
-You need oxygen in the water, so air power and/or surface agitation is needed.
-Leave your filtration in place. If the problem is bacterial, then it will be in the filter as well as the tank. You should monitor the tank for ammonia if possible. Water changes should protect against it. Bbut, if the filter is being harmed by the treatment, you need to know.
-Lower the temp. close to the bottom of the normal range for this fish. Bad bacteria tend not to like cooler temps.
-After day one, change 50% of the water every day. Vac, up the bottom of the tank at this same time.
-Add each day's medication after the above water change.
Get some fake plants, ceramic or rock decor in the tank so the fish can hide and feel safer. Stress weakens the ability of a fish to fight od disease and parasites etc. You can bleach dip the decor afterwards and use it again if the fish dies. Do not use wood.
If I am correct and the fin rot is bacterial, you should see some improvement in a few days. If this is partial, then do a second round of treatment. Improvement means first stopping further fin erosion and, second, seeing some sign of regrowth.
After a month any initial ammonia "burn" should show signs of healing. New growth looks thin & clear or whitish. Yours looks bumpy & as if it's spreading toward the body. I think both Bob & TTA have it right. Have you tested your water? What numbers do you see (not just"ok")?
Large water changes can help a lot but not if bacteria have taken hold. Vacuum too. I would TTA's tratment plan maybe using Maracin 1 & 2 together.
Not to be a buzzkill but I see some lumpiness (?) in the tail too. It may be lyphocytes. There isn't really a treatment if that's it but sometimes it's self-limiting. Clean water & antibiotics can help keep it from getting worse.
My suggestion was based on a few things the OP wrote.
Paraguard is pretty much for external parasites, so is salt among other things and temp. is not a way to treat bacteria. in gacy it is the opposite. None of what has been tried has worked and none of it is an antibiotic.
Using both the Maracyns together is safe according to Fritz. But, it is cheaper if you only need to use one. Under the circumstances I would choose to try Maracyn-2 first. If that did not work, then I would try the Maracyn.
The red text be,ow was made red by me:
About Fritz Maracyn®
Maracyn® is a specially compounded broad-spectrum antibiotic which has been shown to be effective against a number of pathogenic bacteria associated with ornamental fish disease. It was developed for complete but gradual dissolution in water. It is primarily effective against gram-positive pathogenic organisms.
Provides relief from bacterial infections such as body fungus, bacterial gill disease and popeye. Also helpful against mouth fungus, open lesions, hemorrhagic septicemia, and slime disease. Symptoms of possible bacterial infection can include clamped fins, swollen eyes, heavy or rapid breathing, patchy coloration, change in swimming behavior or corner-hiding. Consult a veterinarian or fish expert for assistance in diagnosis of disease causing organisms and treatment options.
All suspected bacterial infections should be treated immediately.
Maracyn® Two powder is a broad-spectrum antibiotic developed for use in aquariums which has been shown to be effective against a number of pathogenic bacteria associated with ornamental fish disease. Due to its unique properties, it is absorbed by the fish through the skin. During the prescribed treatment, the concentration of the active ingredient in the fish fluids rises to a therapeutic level and is available to fight systemic infections susceptible to its action. Maracyn® Two is primarily effective against gram-negative pathogenic organisms.
Provides relief from bacterial infections such as fin and tail rot, popeye, septicemia, dropsy, internal infections, and other secondary infections, such as wounds from injuries or parasites. Symptoms of possible bacterial infection can include clamped fins, general listlessness, degradation of tissue on fins, suppressed appetite or other unusual behavior can be caused by internal infections. All suspected bacterial infections should be treated immediately. Consult a veterinarian or fish expert for assistance in diagnosis of disease causing organisms and treatment options.
Sorry for bad spelling, lymphocytis is what I meant. I often make that mistake, sorry. It's a virus that can spread to other fish even after the original fish looks clear. Look hard at pics...it could be bacterial or fungal...or something else. It's very hard to guess in online pics.
As to what TTA has said, yes, many fish bacterial infections are gram negative, less gram positive but it's hard to say. That's why I suggested using both. But cost can be a factor too. Kanaplex can treat some fungal & bacterial infections but I don't remember which