I was thinking that. Im going to school for fish health and medicine, but I have never had to work with something like a bichir. A white sturgeon is the closest I have ever come, and they are large enough to operate on, lol.My sen has one too. I tried Epsom salts,I feed catfish, didn't help. My wife and some of her colleagues are vet med and believe it is a tumor. Mine has had it since about 3" and is now around 5" he is extremely slow growing he was my first poly that I currently own. Everybody is bigger then him now. Mine eats fine and acts fine. He chases the big female del around trying to get her to mate. When he starts showing deterioration or that he is no longer "happy" I'll worry about it. As for now he's active and happy.
But your right, he seems happy so I guess he must be fine for now.