Red growth on Black Skirt Tetras face

Question
I have 5 black skirt tetra's, 3 lemon tetra's, and 2 albino corydoras catfish in my 10 gallon tank. Today I noticed a strange red growth around one of my black skirt tetra's mouth. It is acting normally and no other fish in my tank have been acting differently or showing the same symptoms as this fish. My tank has no heater, but I do have a filter and add room temperature water to my tank about every week. I feed all of my fish "Tetra Color Tropical Flakes", as recommended by our local fish store. This growth on my fishes face is very upsetting, and I would like to know if there is any way to treat the fish or if this is completely normal. Thank you in advance.  

Answer
Paige,

  It is hard for me to help you without a picture. If you can attach one to this question that would be great. As for the tank, you are very over crowded. With the fish you have in there now you need a minimum size tank of 60 gallons. The lemon fish need 20 gallons alone, 20 gallons for the 5 black skirts, and 20 gallons for the 2 catfish. If you can get a pic of the sick fish I can help you out better.