Black lump on tail of Ember Tetra

[Tombeaux]

• Thanks in advance for your help. I have an ember tetra who came with this black lump on the tail. I bought in a group of 18 so I didn't notice right away. The lump looks like a blood clot, but I have no clue. The fish is in my quarantine tank (parameters below) and so far Erythromycn, Paracleanse, or Ich-X have no effect on the lump. The fish behaves normally and eats, swims fine. So is it a fungus, or a tumor, or just a wonky fish??
• pH - 6.8
• Nitrates <10ppm
• Hardness - soft
   

   
• Nitrite - 0ppm
• Ammonia - 0ppm
• KH/Buffer - about 40ppm
• Water Temperature - 78f

[Colu]

I had an x-ray tetra develope a similar lump and I use Maracyn2 active ingredient is minocycline in food  after 3 days of treatment the lump started to reduce in size after 7 days it was hardly noticeable after  2 weeks I couldn't tell which one had the lump