They are cool. One I bought died right away, but the other one lived for about 6 months. It is actually a funny story. After about 4 months I was out of town and my wife called and said the horseshoe crab died. So, quite awhile later she goes in to look at the tank and there's a horseshoe crab swimming all over. She though I bought a new one, but I hadn't. That crab had buried itself so deep in the substrate that it went unnoticed, even during cleanings, for at least a month. Then, unfortunately, it died shortly thereafter. I guess it had just molted and she thought it was the crab.
CRazy