There are some things you do need to do in order to keep your horseshoe crab alive and healthy.

First you need to set up your marine aquarium correctly and with the right equipment. If you have a marine set up already then adding a horseshoe crab is pretty straight forward. They just need to be acclimatized to the new tank slowly like you would do with any new member to your marine environment.

If you buy a horseshoe crab from a local fish store, be sure not to take all their advice at blindly. I have visited a lot of fish stores and questioned them on keeping horseshoe crabs and the advice they gave was in most cases wrong or incomplete. So beware.