one other thing...not everyone is real concerned about coraline in their tanks. If you would rather see interesting sea life like your urchins, dont worry about it, keep them all and just feed them.
I do not have urchins in my reeftank due to the corals, but I have them in almost every other tank I have because I enjoy them. Funny thing is my reeftank has no coraline algae whatsoever, yet the other tanks do. I have also heard that the urchins will help spread the spores for the coraline. My LFS
keeps an eye out for interesting urchins on his sale list he gets in every week and then just calls and says he has something for me to come see
Last night I saw a huge pretty slate urchin he got for me, but I wont get it til next week as it had just come in. I think they are very interesting creatures.
sorry for rambling...