Please Help Pimple On The Base Of The Tail

[Dr_Shakalu]

Hey everyone. Well, at the base of my flowerhorn's tail there is a bump. It started out as 2 small bumps and now it's only one bigger one. There's salt already in my tank and also I do a 25% water change every other day. Ammonia 0, Nitrites 0, Nitrate 10, PH 7.8. He's eating well and acting like nothing is wrong. He did get into a fight with my other flowerhorn a couple of weeks ago but he is much bigger and beat up the smaller one. They dug down into the gravel and the smaller one was able to squeeze thru. Wasn't expecting that because I changed the substrate to gravel and it used to have small rocks and river rocks so they couldn't move the big river rocks. I don't know if it was from the fight or not. The bump is at the center of the base of the tail on the fin. Please help. Thanks, Ed.