Strange lump help

Hi guys new here...
So a little inside history check I'm not the best with fish... Don't know much about them but got given some boring fish about 6 years ago in a tiny tank now they take up my living room in a 6ft by 2ft tank and I love them!

But I have this one snake fish for about 6months he's had a little lump on his side, the past couple of weeks the lump has really grown and looks like it's about to burst. He's perfectly happy, and so are the other fish he is still eating and swimming ect it's just starting to consern me now. Let me know what you guys think I tried to snap some pictures the best I could!
Thanks in advance!

If it's grown over a 6 month period, it's probably a tumour and there's no treatment for it unless the fish is big and you have a decent fish vet. Even then there is no guarantee the fish will survive any surgery or even if the growth can be removed.

Your best bet is to monitor it and if the fish has trouble swimming or stops eating, then euthanise it.