Hey everyone.
I was "fin"sitting my dad's fish while he's outta town. I noticed his tetra's have this black stuff around their mouth. I couldn't find a camera but managed to find a  picture online that perfectly shows exactly what my dad's fish look like. I cannot figure out what it is or how to treat it. He has about 8 black skirt tetras and only 2 are showing signs of this. It looks like  it's eating away at the lips. Help!

Hi Ash,
I am going over to my dad's tomorrow so I'll bring all my test kits and check out the parameters. I read that link you posted, it mentions I can use medicated food (i have jungle antibacteria pellets) and jungle fungus eliminator in a combo?  I have ample of both on hand if you think that may work. I will try to take them home so I can do all the treating here in my hospital tank and monitor their progress.

Hi Ash,
So I did check dad's water and was in shock by the nitrates,gh and phosphates. All of them where off the chart. So did a 25% water change gravel vacuumed, and started feeding the antibiotic pellets yesterday. Dad is now back in town so I explained the nitrogen cycle and advised him to do more frequent water changes to bring down the nitrates. I'm surprised he hasn't had a huge algae bloom yet or lost fish. Told him to do 25% water change every 2 days until he get under 20ppm. Fingers crossed with the food and clean water it will heal itself.  Hard to try to teach someone who's been keeping fish quite successfully for over 40+ years that what they are doing isn't quite right.

Went over today to do another 25-30% water change and looks like it is spreading to the other black skirt tetra's. He has 8 and now looks like 5 are infected. I told him he should  keep feeding the  medicated food  for the full 10 days because of all the other fish in the tank. It doesn't seem to be affecting his guppies or neon tetra's...yet. He did buy some new neons and 4 black skirt tetras about 2 months ago.  He never quarantines cuz he never gets illnesses. I told him to start using water conditioner almost 6 months ago (since he normally just throws water into the tank) and now I feel like his wife is blaming me because now the fish are sick. "they were never sick before..blah blah blah".
I'm disgusted by the colour of the nitrates..it's a very dark cherry Kool-aid type colour which I can't even find on my Nitrate colour chart. Poor little fishies.

With those diseased mouths are they even able to eat the antibiotic food?
It can be really difficult when others are so set in their ways. Just know that you've done a really good thing by trying.
Barb

Strangely yes they can still eat. They do not appear to be emaciated. Colours are good,  they're active and chasing each other. It's just their mouths that appear to be rotting away like some type of flesh eating bacteria.  I'd expect some type of fin rot but..they look good other than their mouths.

The infection is likely flexibacter, sometimes known as "Mouth Fungus".