I will never buy another convict tang again. I bought one when I first started. It was cool, but went crazy for some reason. Just darted back and forth in a frenzy, then act calm again, and then crazy again. It died in a copper treatment. So I bought another one. I was crazy from the start. Kept all the other fish hiding. It looked like there were no fish in the tank, cause it sared them so much. I sold it and I guess it immediately killed one of the guy's other tangs! And a month later jumped out committing suicide. My guess is that they like to be in schools. But I have seen a few pictures of them in other people's tanks and they look just fine. I wish mine had acted normal, cause I really like the way they look.
Let's see, most of the stuff I chose, I just would never buy. Either cause I don't think it would be a good idea or I think they are ugly.
Oh and did you mean to put "arrow crab" instead of "spider crab"? My dad had two arrow crabs in his tank. He liked them, but I don't.
And for you people who chose emerald crabs, did you have a bad experience with them? I have about 8 in my tank and so far they've been well mannered.