Pretty much what
kno4te
said, i didnt see the bump either but based on the dark belly and washed out colors i’m guessing something is stressing it out...
PufferCorps
i’m also going to presume you eliminated tank parameters being out of sync???
If so, i would maybe look into some of the following:
1. Are the tankmates possibly picking on the smaller puffer when you’re not around / after lights out???
2. Is there any other signs the puffer maybe sick at all (other than the bump)...
3. Could some of the food you fed your puffer have been contaminated with something by chance???
4. This ones a real reach, but any chance of stray electrical current??? Although this one would more than likely effect the other puffers too...
5. If you feed hard shelled foods maybe something got lodged inside your puffer causing it discomfort???
6. I would also keep an eye on the puffer to make sure its pooping normally, coz it may even be a simple case of constipation and just requires some epsom salt to help him do his business...
7. FINALLY, it could even be a matter of your puffer just sulking, so depending how long hes been like he is in the video, it may or may not be much to worry about...
But otherwise, see how the general cure goes first and keep an eye out for any changes...