[MJvanColler]

My one Cory has a light pink mark just below his fin, only on one side of his body. He is still active and swims to the top for air but he doesn't scavange like the other Corries. The other fish (3 Corries and 6 neon tetras) seem healthy.
I've had him/her for at least three months now and do about 25% water changes every week.
The only change is that recently we've been having power outages, about 2hours at a time, were the temperature drops with about 2°C but goes back to normal once the power comes back on.
I don't have an extra heater and filter to make a long term quarantine tank.

I've uploaded a photo of his/her pink mark

 

[Discusluv]

Hi,
I am sorry that your fish isnt doing well.

I need more specific information:

The first thing you look at when fish show symptoms of illness is to do water tests. Most of the issues with fish that are in our tanks for awhile have to do with lowering water quality.
*What is your ammonia, nitrite, nitrate reading? I need the actual numbers, please.

* Have you had any other deaths in tank in last 6 weeks?
*Have you added any other fish in last 6 weeks?
*Tank size?
*Temperature?

*Do you notice that the fish is lethargic? Like if you put your hand in tank does the fish swim away quickly as normal or does it slowly swim away.

*What do you feed and how often?

 

[MJvanColler]

Nitrite 0 Nitrate 0 and Amonia 57.
I got 6 neons and the two peppered/leopard corries along with another plant about 3 months ago. No deaths. They were put in an established tank with two albino corries. I feed them a Dennerle cookie once a day (which I split cause the albinos are a bit bigger and tend to hog the food) , and vary the food I give to the neons although all specifically for tetras.
I added bacteria to help get the Amonia down.

Sorry, tank size is 30l and the temperature I try to keep at 23 or 24°C for the neons.

Last thing my bad. He reacts well to other fish coming near or when I'm close to the glass and he still darts to the top for air no more than the other corries. He doesn't dig around the substrate much though

 

[Discusluv]

The issue is most likely due to ammonia. I usually deal with ammonia totals in PPM so unsure what a measure of 57 signifies.
Regardless, even slight traces of ammonia can stress a fish and make it vulnerable to secondary bacterial infections.
Have you checked your water parameters coming from your tap: ammonia, nitrite, nitrates? Its a good idea to get a baseline number of what you are dealing with prior to going into your tank.

How are you testing your water parameters?. It is very unusual that a cycled aquarium have 0 nitrates unless heavily planted. The strips are very unreliable, so I would advise to get an API liquid test kit if using strips.

Treatment: First thing I would up the amount of water changes you are doing in the short and long-term to control rising ammonia.

Short term: I would do a 50% water change today and then retest to see where you are with ammonia. If ammonia not at zero, do another 50% water change.
Long term: Do 50% water changes weekly, obviously the 25% you are doing is not enough.

Improving water quality may be enough to aid your cory. Traces of ammonia suppress immune system. Clean water may be all you need to remedy this situation.