I will not guess as to the specific issue, but there are a few things of concern here that will weaken the fish, making it more susceptible to other issues.
First is the gravel. Cories must have sand. The problems with gravel are significant. They cannot filter feed as they expect, the large gravel grains cause more bacterial problems which are critical because cories spend so much time on the substrate, and the gravel is rougher than sand. There is a good chance the issue here is bacterial, but there are other possibles that other members with more experience of these can assist.
Cories must be in groups. This is something they expect, and will suffer stress without several of them. Stress is responsible for 90% of all aquarium fish disease, so this is important. However, before acquiring more cories, the gravel problem must be fixed.
Nitrate is high. Anything over 20 is not recommended, and as low (close to zero) should be the aim. Assuming there is no nitrate in your water, this should be easy to achieve with regular substantial water changes, not overfeeding, cleaning the filter and substrate at water changes. If nitrates are present in the tap water, that is another issue.