Cory Cat Developed Inflamed Red Patch - Help Diagnose Please

[JillianTruant]

my albino cory cat has always had a little bit of pinkish discoloration behind his pectoral fin on one side. it was barely noticeable when I got him about 3 months ago, but since then it has gradually grown in size and now it has turned red and almost seems to have a piece or skin or something protruding from it. it was just pinkish two days ago, and seems to have gone from pink to angry red overnight.

I have attached the best pictures I can get. he is still very lively and his behavior hasn't changed at all. he is still full of energy, schools with my other corys, eats well, and is not exhibiting any signs of distress beside not moving that particular fin very much.

for reference, I have a 20gal tank. I currently have a pair of dwarf gourami, five small neon tetra, and three cory cats. none of the other fish have any injuries or signs of parasites, disease, etc. my water parameters are excellent; I use the API master test kit. I do a 10-20% water change 3 or 4 times a week. everyone else is happy and healthy. I have a gravel substrate and the tank is not planted.

does anyone have any idea what might be wrong with him? it is definitely not a sudden injury, as he's had a discoloration there for months.

 

[Colin_T]

It's a cyst or blood blister, probably a cyst. There's not much you can do about it. When it gets too big and the fish can't swim properly, or if the fish stops eating, then euthanise it.