30 Gallon Tank My corydoras woke up with two identical blood marks

Hello, does anyone could give me ab idea how in the world my corydoras woke up one morning with identical red marks on the sides, that look like blood.
He is the only one with that, i am not sure if that is some known issue with pepper Corydoras? Or some other sickness? It is not red blotch for sure though.
Also all my water parameters are within range, in fact im a psycho of keeping my water parameters at perfection, doing my maintenances and filters are perfect.
Also, notice that i have been using the paracas rocks for years as substrate, so this substrate is 100% round for them, not a single scratchy rock on the entire tank. I do not have ornaments and just plants.
They live only with peaceful platys and no one bullys my guys.

Also, his attitude and behavior is normal as usual. He seems to be fine, eating fine and scavenging without problems.
Still i am afraid that this could end up in a infection or get worst. Your help is highly appreciated

 

I think it wedged itself in the substrate trying to get the food that got out of reach. The substrate isn't that suitable for corys for that reason.

 

Thanks , unfortunately i dont think the substrate had anything to do with it. Like I said, i have been using this paracas substrate for many years with my Corydoras and they have always lived fine and healthy. Im pretty sure it gotta be something else. I just can't tell if it is a desease or he really hurt himself

 

Could he have wedged himself behind your heater? or behind the uptake tube of your HOB filter. Maybe down behind the strainer and scraped himself?

 

It definitely looks like a physical wound. The way it is on both sides would mean it had to have got stuck in something (unless you have a fish big enough to try eat your cory which seems unlikely). Even with a rounded/smooth substrate, it's still possible a cory got down into the rocks and injured itself. Otherwise heater, filter, suction cup or wood etc...