Either a nematode bump or an old injury would be my guess. Keep an eye on it and if it moves, please, please, record the change photographically. My mbu had at least a couple, and periodically they would vanish from sight, then later reappear elsewhere.
I got her at about 7-8 inches and she has always had it. I wil keep my eye on it. Best fish I ever got! I love her to pieces - so much so that I often get into trouble with my other half. "what do you mean, you like a fish more than me?"
Oh well, she has a box fish and a cow fish that I know she is very fond of!
Once again, the four toothed fish has got me addicted.
My Puffers: Puffers I have kept: T lineatus T miurus A modestus T biocellatus C valentini C coranata C papuan C lorteti DPs suvattii burrfish T niphobles
I will ask her if I can touch it.
She said 'sure!' Doesn't feel hard - more fatty. It doesn't grow/shrink/move etc.
I still think it's all preparation for when she grows the unicorn horn and becomes the most powerful puffer in the world!
Once again, the four toothed fish has got me addicted.
My mbu's nematode lumps rarely moved - one that never moved (on the back, not far in front of the dorsal fin) I suspect was dead and organized as scar tissue internally.