Ammonia “burn” at that level will cause significant to complete fin disintegration
It looks as if there was a physical impact (which seems unlikely ... unless fish somehow leapt out of water, hitting hardscape, floor etc) but internal bacterial infection seems far more likely (the red appears from localized hemorrhage and base of the caudal fin is not uncommon), it may also be viral in origin (viral hemorrhagic septicaemia)
Unfortunately none of the above are able to be treated in a home aquarium but supportive care can assist fish in healing
Daily water change (50% if you already change that amount weekly, otherwise begin with 25% daily for a 5 days, then 50% daily - the large water changes will remove waterborne pathogens, eg, bacteria, virus particles etc
Vacuum substrate to remove debris etc (again this can be a significant source of bacteria etc)
Wipe down glass (biofilm including more bacteria)
Reduce temperature - bacterial infections may respond to lowered temperatures
eg, Corydoras aeneus
Temperature:
21 – 27 °C
So depending upon other fish in tank, lower temperature by 1-2* daily
Do not add salt for Corydoras, note that salt acts as an irritant to fish, stimulating increased production of skin secretions - which may or may not be beneficial
Salt may also assist in alleviating symptoms of nitrite poisoning ... but water changes would also be effective
As you’ve had the Cory’s for a couple years, the condition would have developed in your tank ... I suspect the gravel contains high levels of bacteria
- is there obvious debris in syphon tube when you vacuum the gravel?