Problem with Gold Laser Cory

[seaweeds]

I bought 4 gold laser cory online and noticed this on one of my corydoras. I am new to aquarium hobby and have no idea what this is. It looks like cracks under the skin. After 2 weeks, I also noticed it on my other corydoras too but not as severe as this one on the same exact location. I searched online but could not find an answer. I have used ichx, api erythromycin, api general cure, and seachem polyguard but does not seem to heal. Does this have to do with temp because I am quarantining them at slightly higher temp. Does anyone know what's the problem with my corydoras?

 

[FranciscoB]

It will help to post additional pictures, different views, and to describe in detail what you are seeing that believe is abnormal or a problem. By all means, stop adding medicines and treatments unless you have a good diagnosis of the problem. In the single image shown, I don’t see anything wrong. Good luck!

 

[seaweeds]

It look like a white spot near the dorsal fin on its back with 4 lines that look like an X. At first I thought it was some kind of parasite attached to the back because when I first acclimate the fish and introduced to quarantine tank, I noticed something that look like a shrimp the size of a grain of rice. On closer inspection, it looked like cracks under the skin of the corydoras. Not sure what caused it and how to fix.

 

[CassCats]

Add some heavy tannins to the tank, after doing a good water change. To me, it seems to be the start of a minor bacterial infection and it's enough that quality tannins may help heal it.