Lump on dwarf rainbowfish

[MaxNudeHoovering]

Hi
This is my first post on this forum, I have a 200l tropical aquarium, most of the fish I have are rainbows. They're mostly peaceful but become boisterous at times. But over the past year I've had 2 dwarfs that have got this lump, unfortunately the other died. The fish isn't acting distressed or out of the normal, he's also eating very well.

I'm unsure if it's from a nip that's got infected or something more serious (lympocystis or so). The tank is very adequately filtered and has a strong water flow, I clean and change 40 percent of the water weekly. I'm seeking advice on what to do with the fish? I tried anti bacterial/fungus treatments on the previous rainbow when I had a spare quarantine tank but it was no help.
Thanks
Max

 

[Mark Z.]

I have an Australian rainbow with a similar lump; along its lower body near the fin. I'll be interested to see if anyone knows of a treatment. My fish seems healthy otherwise.