red sore

My betta Valentino had a little bump near the base of his tail a couple days ago, and it's now an open sore that I'm pretty sure is an ulcer. He's never had any health problems before, and the only thing I can guess that would have caused this is the stress of moving into a divided tank? How...

My fish suddenly (seemingly overnight) developed a red sore / blister on his anal fin. I'm having a difficult time remedying this and can't seem to find any solid information on the internet about this problem. If you have any experience with this problem please let me know. I appreciate any...