Tuxedo urchin losing feet? HELP!!!!

[Marinefish]

Hi, I recently purchased a tuxedo urchin, and he did fine for about a month, then he started losing "feet". At first it was just a few missing from his top, now he has none on the top, and he can't move because he has too few feet on his underside. I can think of two things: one, my 3/4" randall's pistol shrimp likes sushi, or two, my clown goby developed an odd growth 6 weeks ago, so I called three pet stores (all good ones), and got a product called Metronidazole that is reefsafe, could this be it? The urchin is definitely alive (his spines move toward anything that touches him), and he's on top of some nori, but i'm flabbergastoozled as to what's going on. HELP!!!

[Reefing Madness]

Welp, when they get stressed out they lose their spines, like when a fish picks at them. Also lose them because....... Why did you put the Metronidazole in the tank? Fish got something? Infection? Ich? Intestinal parasite?