Urchin troubles

[TAZ]

Hi all,

A couple months ago we added an urchin to our tank, for the longest time he has been doing great. Moved around the tank a lot, and seemed fine. However, over the past week he slowly started to lose all his spikes, and then this morning I went to check on him and he had died. Has anyone had any experiences like this with an urchin, any idea what might have happened. The first day I noticed spikes falling off I ran some water tests, I also ran tests on my water this morning both with the same readings:

Calcium (ppm): 420
Ammonia (ppm): 0
Nitrite (ppm): 0
Nitrate (ppm): 0
Phosphate (ppm): 0.00

On the weekend I did a 10% water change, and am in the process of mixing up another batch of water to do another water change. All our snails, our cleaner shrimp and all the fish are doing great, so I am stumped as to why the Urchin died?

Thanks