I have two in my tank for about a year now. One is a mine urchin and the other is a blue tuxedo urchin. They seem happy at least I can tell by the tuxedo one because the one characteristic this urchin displays is carriying loose pebbles and whatever it can find in the sand around on itself in order to be camouflaged This is how you they are healthy, at least it's what I have read. I have yet to feed them personally. They feed themselves with detritus and coraline. They are constantly grazing on the rocks and glass. By the way I have a fwlr & mobile invertabrates
i have two blue tuxedos. i dont feed them for the same reason i dontfeed my snails - their whole purpose in my tank is to eat up all the junk, wouldnt want them to get full on 'real food'