Hello. I am new to the saltwater tank business. On my my black rock urchins is sick. I'm not sure why. I got them 3 days ago. Yesterday I noticed that one of the black rock urchins I got wasn't moving from one spot. He still looked healthy, but he wasn't moving. This morning I saw him and he lost quite a few spines. I put him on the rocks with the most algal growth and I also put a little bit of seaweed near him as well in case he wasn't eating. The other urchin looks perfectly fine, though he hasn't move a lot either, he moves small amounts each day on the rock. I also have a brittle star and it looks fine as well. All the parameters in the water look great and the salinity is perfect as well as temperature. I have a 55 gallon tank with a 6 gallon canister filter. I'm not sure whats wrong with him but I would really appreciate some help.