My Corydora shows two symmetrical blood marks on his side. Why? Please help

Hello, does anyone could give me ab idea how in the world my Corydora 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

It might just be his organs/brains.

What will cause an infection however, is having gravel as a substrate. Corydoras need sand so they can properly filter feed. When there is gravel present, they will injure their barbels, which will cause them to become infected.

It's a physical injury caused by something in the tank. The fish might have got caught behind or under something and hurt itself. Its barbels are also looking a little worn.

Do a big (75%) water change and gravel clean the substrate every day for a week. This will reduce the number of disease organisms in the water and reduce the chance of infection.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Clean the filter too. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the lawn.

Add 1 heaped tablespoon of rock salt for every 20 litres of tank water. If it looks worse in 2 days time, add a second dose so there is 2 heaped tablespoons of salt per 20 litres of water. Keep salt in the tank for 2 weeks.

When you do water changes, add salt to the new water before adding it to the tank so the salinity (salt level) in the aquarium remains constant.

PheonixKingZ said:

It might just be his organs/brains.

What will cause an infection however, is having gravel as a substrate. Corydoras need sand so they can properly filter feed. When there is gravel present, they will injure their barbels, which will cause them to become infected.

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Hi . Thanks for the advice. In fact, i have this paracas substrate for quite long time, if not years already and my Corys never had an issue. Also, here in Perú , Corydoras live in even sharper rocks and stones than this, in the real environment in our Amazonas river. My paracas substrate is totally round at all, no sharpies anywhere. The only new change i have done is that i added the fluval intake and output hoses inside the tank. Im not sure if he could have gotten stuck between somewhere in the intake clips or i dont know what else to think.

Cover the intake hose with a pre filter sponge to reduce the chance of fish getting stuck.

jclv said:

does anyone could give me ab idea how in the world my Corydora woke up one morning with identical red marks on the sides

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It's an injury as @Colin_T stated as my gold pleco i had caught my albino on the eye and side and my cory died the same night but when i removed him the mark was the same as yours but from the eye to the body. I saw the attack but never knew he'd cut my cory.

Tez_20 said:

It's an injury as @Colin_T stated as my gold pleco i had caught my albino on the eye and side and my cory died the same night but when i removed him the mark was the same as yours but from the eye to the body. I saw the attack but never knew he'd cut my cory.

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Thanks. Now that you mentioned that, i see sometimes a couple of my platys get a bit excited about food so they once in awhile try to bite my beautiful and peaceful Corys , to try and get the food first, but in all these years never seen a single issue like that in my Corys. I have them like 3 years by now, and they have been all fine. It is just seems weird to me that recently i just installed the new fluval and this happened.

Naughts said:

Cover the intake hose with a pre filter sponge to reduce the chance of fish getting stuck.

Click to expand...

That is a nice option. In fact i had two baby platies around recently and this fluval sucks way more than my old sunsun canister

it might have been sucked onto the filter.

Colin_T said:

It's a physical injury caused by something in the tank. The fish might have got caught behind or under something and hurt itself. Its barbels are also looking a little worn.

Do a big (75%) water change and gravel clean the substrate every day for a week. This will reduce the number of disease organisms in the water and reduce the chance of infection.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Clean the filter too. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the lawn.

Add 1 heaped tablespoon of rock salt for every 20 litres of tank water. If it looks worse in 2 days time, add a second dose so there is 2 heaped tablespoons of salt per 20 litres of water. Keep salt in the tank for 2 weeks.

When you do water changes, add salt to the new water before adding it to the tank so the salinity (salt level) in the aquarium remains constant.

Click to expand...

Yes, i just did the water changes these days and more often than usual. Although i am mot sure about the salt for Corydoras. I tried that long time ago and Corydoras died after a week or so, due to the salt. I think is too strong for them

jclv said:

It is just seems weird to me that recently i just installed the new fluval and this happened.

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@Colin_T carries a good point about being sucked on the filter and it is a bit funny after installing the Fluval.