Unknown blemish on caudal fin

[michiwop]

Hello All, I am new to the forum and I'm hoping someone out there can tell me what this "blemish" is on my male Cac's caudal fin. As far as I can tell, this lump is not affecting his routine at all. He eats very well and continues to be one of the most prolific fish I've had the pleasure of keeping. I first noticed the lump about 2 months ago and since then it hasn't changed in size or appearance that I can tell. I'm sorry the pictures are dark, I will try to get some better pics this evening.

I've been breeding cacs for years now and I've never seen anything like this. Any help or information anyone has would be greatly appreciated!