I just got one today after a ton of research.
And your research told you it's OK to get 1?
Critter of the Week: Limulus polyphemus (Horseshoe Crab)
"Horseshoe crabs should only be kept in large (several hundred gallon
) mature systems. Best case scenario would be something like a biotope, mimicking the crab’s natural environment of mudflats or sandbeds with sea grasses and sp**** rock. Each crab needs a minimum of 10+ square feet of open (mature) sandbed, at a minimum depth of 6”.
Not suited to reef tanks because of the crab’s need for cooler water (68 – 79 degrees), open sand areas, and their tendency to knock things about. In addition, they have an unfortunate tendency to get wedged into the rockwork/aquascapes so popular in reef tanks."