Unknown Red Mark on Corydora

[Anonfish4]

I was watching my fish last night as discovered a red mark/few red lines on my corydoras behind the gills and pectoral fin. It's only on the left side. I'll try to attach a picture after school. The fish doesn't flip/flash and seems to be swimming normally. Any idea as to what it could be?

UPDATE:
Picture


The mark isn't as red as it was last night, but it's still there

 

[Sofiafish17]

Hmmm can you give us test results when you get home too?

 

[Anonfish4]

Hmmm can you give us test results when you get home too?

Yeah absolutely. I took one a few days ago and nothing was wrong but I'll definitely do water tests. Anything specific or just ammonia, nitrite, and nitrate?

 

[DoubleDutch]

And a picture please

 

[Anonfish4]

And a picture please

The red mark doesn't look as bad as it did last night, and now it's just the small spot above the pectoral fin. It's still a cause for concern in my books though. I thought it was red blotch but I might be wrong.

Yeah absolutely. I took one a few days ago and nothing was wrong but I'll definitely do water tests. Anything specific or just ammonia, nitrite, and nitrate?

Hmmm can you give us test results when you get home too?

I used a freshwater master kit by API in case that's important.
Nitrates: between 0ppm -5.0ppm
Nitrites: 0ppm
Ammonia: 0ppm
I did wait the 5 minutes for colour to develop

 

[DoubleDutch]

I think it is caised by the substrate.
The Cory will search for foodparts between the pebbles and than hurt itself.

 

[Sofiafish17]

Corydoras only do well on a sand substrate. I looked and saw you other post as well, and this is why they were pacing and missing barbel as well. you should switch over to sand. The easiest way to do this imo is to just get the sand and then nice rinsed, dump it on top of the gravel. You should take out all decorations and put them in a bucket of water so they don’t dry off, and hold the fish in the same bucket of water while you work. If you rinse the sand and do it well there should be a bit of cloudiness but not too bad, and it will clear up when you turn the filter back on. oh yeah, be sure to turn the filter off while you work too so it doesn’t get gummed up with it sand particles. A sponge filter will be ideal if you have one. In the end you should ideally have at least 3/4 of an inch. Then you can just put any driftwood, decorations, etc back in and replant the plants. You will want to do a water change during this (or at least I would just because it’s easier once you’ve already been in the tank but you don’t have too) and even if you don’t do a water change, acclimate the fish like they are new fish. If the tank is established, you should not loose your cycle because lots of the beneficial bacteria will be in your filter. The sand will be much softer on the cories bellies and won’t scrape them up like gravel. Oh yeah, and maybe clean a clear path from where you are rinsing the sand and to the tank, so it will be easier to vacuum. Don’t just try to step around clutter, I did that once and kept finding sand in my laundry

 

[Anonfish4]

Corydoras only do well on a sand substrate. I looked and saw you other post as well, and this is why they were pacing and missing barbel as well. you should switch over to sand. The easiest way to do this imo is to just get the sand and then nice rinsed, dump it on top of the gravel. You should take out all decorations and put them in a bucket of water so they don’t dry off, and hold the fish in the same bucket of water while you work. If you rinse the sand and do it well there should be a bit of cloudiness but not too bad, and it will clear up when you turn the filter back on. oh yeah, be sure to turn the filter off while you work too so it doesn’t get gummed up with it sand particles. A sponge filter will be ideal if you have one. In the end you should ideally have at least 3/4 of an inch. Then you can just put any driftwood, decorations, etc back in and replant the plants. You will want to do a water change during this (or at least I would just because it’s easier once you’ve already been in the tank but you don’t have too) and even if you don’t do a water change, acclimate the fish like they are new fish. If the tank is established, you should not loose your cycle because lots of the beneficial bacteria will be in your filter. The sand will be much softer on the cories bellies and won’t scrape them up like gravel. Oh yeah, and maybe clean a clear path from where you are rinsing the sand and to the tank, so it will be easier to vacuum. Don’t just try to step around clutter, I did that once and kept finding sand in my laundry

Thanks! I'll try and get sand to put in the tank this weekend. Is there any brand you recommend? Will siphoning the tank be different? For example when I vacuum the gravel in my tank I go and vacuum the gravel itself. Would I still do this with the sand or would I just siphon out water? I'm sorry if this this a lot to ask but I'm new the sand and want to make sure I do it right.