Help with my Corydora catfish!

[PhishBoy]

Intro:
Hi everyone thank you for taking the time to read my first post. This isn't my first forum experience and I am well aware of the search function. However I wanted to post this asap and hopefully someone can help me faster then me looking for the answer.
The problem:
I noticed a couple days ago that one of my cories has a weird white smudge on his face. His dorsal fin is also limp and he just lays around all day and hides when the lights are on; while the other two play and swim around. This is my first real tank and I am worried he might get be sick.
It is a 10g tank with only 3 corydoras so far. I have had them for a little over 3 months with 3 week water changes and 4 week filter changes. I'll include pictures for reference. Please let me know if it is bacterial, fungal, or maybe he is just an odd ball. I'm really worried as any noob would be.

 

[Colin_T]

it's probably just colouration.
check the water quality for ammonia, nitrite, nitrate and pH.

do a 75% water change and gravel clean the substrate eveyr day for a couple of weeks or until the problem is identified.
make sure any new water is free of chlorine/ chloramine before it's added to the tank.

have you added anything new to the tank in the last few weeks?

is the fish still eating normally?

 

[PhishBoy]

Hi Colin_T,
Thanks for your quick reply. I followed your instructions for a week and he seems to be doing better now. I actually see him swimming around and eating now so that's a plus.

 

[JxsPxxle]

Hi Colin_T,
Thanks for your quick reply. I followed your instructions for a week and he seems to be doing better now. I actually see him swimming around and eating now so that's a plus.

Now that this problem has been addressed, you have a stocking issue. Corys should be in group of 6-8 at a minimum but 10 is ideal. A group is three is not really enough, but you could fit 6-8 dwarf corydoras in your tank if it is a breeder.