I've had Derp for about 4-5 months roughly, he's been eating (as far as I can tell, I see him eat sometimes when I watch), He is very active and dives and hides when I tap the glass. he's had this lump on his butt for most of the time I've had him and I noticed today it's twice as big as it used to be! My boyfriend read on-line to try and see if it would come off by wiping it with a q-tip since it might be built up dead skin (he saw it on a forum) but nothing happened. It moved and that was it. I'm not sure if I was supposed to but I picked him up out of his tank, which to total shock I didn't have much of a problem doing to look at his nub. The nub did move when I rubbed it so... I;m not sure what do to do with it... Here are some pictures.

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