Keeping horseshoe crabs is fairly easy. They are pretty hardy and tolerate a pretty wide tolerance of salinity.
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.
I caught a horseshoe crab off a pier tonight and a person said the tail was poisonous so they cut it off. The crab later emitted a blue like liquid from the open cut from the tail removal. The person said this blue is very poisinous. It was caught off a pier in the Chesapeake Bay. we were going to eat it like other crab but since they are poisonous then we will not eat it. How long will it live in a 5 gallon bucket?
There is no part of a horseshoe crab that is poisonous. The tail is only for turning around and righting themselves when they are upside down. The blood has copper in it that turns the blood a dark steel blue color. Some people in other parts of the world do eat them, but there is just not much meat to make it worthwhile. However, the blood is worth more than gold. It is used to make medicines and test for certain diseases such as spinal mengengitis. When the blood is collected, the animal is not killed in the process. They remove about 25% of the blood and set the animal free.