If its a polyphemous (sp?) Than it will grow to about 4 feet in length. A reccomended tank size (not much on them) was 500 gal. And even at that was reported a low survival rate, as its just not the space they need to roam and forage for mullusks. I'm just concerned for the health of the animal. The polyphemous (again sp?) Can also go nearly a year without eating, and it could be 2 oe even 4 molts before you realize he's dead.
As for my fish, all are more than capable of living long, healthy, happy lives, as they don't need the space that a lineus polyphemus horseshoe crab needs. The smallest of the horseshoe crabs get about 1 to 2 feet, and that's still a need for a bigger tank that what you have...I'm just trying to inform you of the negatives of these fish. If your able to get a 500+ gal tank, and properly take care of it, great, I'm just hoping your taking into consideration the responsibility of the animal. Also note that one species, from thiland lays eggs that contain some sort of toxin, and can kill people, not to mention your curious livestock..