I did a little research and this is what I found " The Horseshoe Crab is very strange in appearance and can be unique captives for an aquarium. Horseshoe Crabs are bottom dwellers, and in the wild usually burrow in mud or sand flats in tidal areas or estuaries. In the aquarium, your Horseshoe Crab will generally do best in a sandy substrate. They are helpful in cleaning an aquarium or sifting sand. In the substrate, the Horseshoe Crab will move about, scavengiong detritus or other materials that have fallen to the bottom. Algae, smaller bivalves, worms, and bits of fish are the mainstays of a Horseshoe Crab's diet. To grow, the Horseshoe Crab sheds a layer of its carapace, or molts. This process is accomplished at longer and longer intervals as the Horseshoe Crab ages. In general, Horseshoe Crabs are not aggressive; they are scavengers, not hunters." I also read that they do not have jaws and that they digest small bits of food that they kind of grind up in their stomach which has some sand and gravel in it. this is something else I found that I thought was important. "One final word: This ancient animal is in danger of becoming extinct."
A lot of people suggest that this is one of the creatures that are best left in the ocean. they can grow very big and live for up to 9 years or longer.
When I first started out I bought one and it did die within a short period of time. of course I was new and didnt know what I was doing. IMHO they are best left in the sea, I have not read of a lot of success stories of people having them in their aquarium but I would be interested to read if anyone has been able to keep them over a long period of time.
Sorry this was so long, I just wanted to give you some facts that I researched and personal experience.