Red spot on Pygmy Corydoras

Hello! This is my first time posting and my first time owning fish with knowledge of cycling. My 10 gallon tank has been cycling for a month and a week and I added a female betta and six corydoras two weeks ago.
I picked up three more corys and it seemed to boost all the cory’s confidence in the tank.
I have noticed that one of them, which I believe is one of the newer ones, has been hovering mid level for extended periods of time, swimming in little circles.
Looking at it closer today, I noticed a little red dot right under its tail. My betta has been pretty chill with the Cory, only occasionally bullying them out of the way if she thinks they’re eating something she wants. She’s completely ignored this one floating mid level though and hasn’t nipped at it at all from what I’ve seen. I’m worried it might be some sort of fungus and am wondering if I should isolate it? One of the other corys kind of joined it but soon went back to normal foraging and none of the others are acting this way. I tested for ammonia and they’re under 0.25. I plan to do a partial water change today but wanted to know if I should take further action or just watch to see if it improves. Sorry for the poor photos, it just won’t stop moving

 

Hi , I keep pygmy Cory's myself they are a great fish. Don't worrying about them swimming mid water these are one of the types of Cory's that will swim mid water and won't always stay on the bottom. They will also swim to the top of the water and grab a gulp of air and swim back down. Pygmy's are quite a sensitive fish especially in new aquariums, it is quite common to lose one or two after purchasing but once established in aquarium you shouldn't have issues. They also have such tiny mouths food can be an issue, I feed live baby brine shrimp 3 times a week to fatten them up. One disease that Pygmy Cory are particularly susceptible to is red blotch disease. This is characterized by bloody sores on their body (usually the belly). It occurs when the fish are stressed by environmental conditions. The water parameters need to be maintained so perform water changes each week. Low oxygen is thought to be the main factor. This is because they are new and still adapting to aquarium so just keep the water fresh and keep an eye on Betta aggression. I have kept these with Bettas no problem but all Bettas have different personalities and some more aggressive than others .

 

I haven't run across this to treat it, by which I mean I've seen fish with red spots like this in stores, but have not had it happen in one of my tanks.

Best guess (and it is just a guess) would be something bacterial like septicaemia. I'd treat it with something appropriate to gram-negative bacterial infection. Up to you whether you want to remove it for treatment - there's always the balance between adding stress by removal, vs the chance that it may die and other fish eat from the carcass before you find it and so become infected as well.

Given that you do have a betta in there, I'd probably remove the sick fish just to keep the betta from bothering it. Even if it's not bothering it now, if it gets weaker she probably will.

 

Thank you for the advice! I’ve separated him into a small spare tank I had as I noticed this morning that she had in fact taken some nips out of his tail. The red spot had also gotten bigger. I have him in a temporary tank of 1.5 gallons with an air stone and plan on dosing him with some Melafix to see if he improves. He’s swimming oddly with his back end a little lower then his front, which he had been doing before but not so severely, but I’m not sure if it’s from infection or injury. I’m considering now if I should separate the betta from the others. She seems pretty okay with the littles for the most part and I wonder if this one just came ill to begin with. Thank you again for the help!

 

If it's already worse, Melafix isn't going to help. I would treat with Furan-2, or Kanaplex, or Maracyn-2. If it is septacaemia, a gentle anti-bacterial like Melafix isn't going to be strong enough, it needs actual antibiotic.

 

Makes sense — I’m going to see if I can pick one of these up today at a LFS. Do you think it’d be a good idea to give him Melafix for now or would it be best just to wait until I can get something stronger? I know either way there’s no guarantee he’ll make it as part of why I separated him was, as you mentioned, if he does die he doesn’t infect everyone else, but would giving him Melafix for now give him a fighting chance?

Thank you for the help! I ended up separating him as I noticed this morning that the red dot was larger and that the betta had been nipping at him. He’s also swimming with his tail a little lower then his body, which I’d noticed before but it now seems more pronounced. He’s in a temporary 1.5 gallon with an airstone. I’m thinking of dosing him with some antibiotic to see if it clears up. I was thinking Melafix but since it seems to be getting worse, someone suggested using something stronger. It might take me a while to get something stronger so I was thinking of using Melafix for now then switching. Do you think that would be a good idea or should I just let him rest up and regain strength to see how he does?

 

I'd probably wait. There are things that Melafix is good for, but internal infections it's really not going to help. Just like using Tea-Tree oil with humans - for something skin-related it's useful, but for a kidney infection it's not going to help.

 

Did you read my post above? Bacterial infection: Bacterial infection is caused by the presence of bacteria in the tank, it can be difficult to tell the actual bacteria causing the harm. A good example is the Red blotch disease. Symptoms include bloating, red ulcers, red streaks.

This infection can be treated with a broad-spectrum antibacterial medication. NB: Some assumed bacterial infection can actually be a fungal infection when at crossroads you can treat with Ich X and Erythromycin.

What is the flow rate like in your tank? Do you have enough surface agitation? Keep water clean this is probably from poor water quality in shipment/store. Is the tank you moved him to cycled?