Keep in mind also that horseshoe crabs should NOT be placed in a tank using a deep sand bed - or a sand bed as part of the biofilter, as they do eat the beneficial creatures in the sand bed. Obviously, due to burrowing, they also disrupt any stratification of the sand that gives rise to nitrate reduction. They are commonly blown around by too strong of a flow in our tanks, and end up sucked onto powerheads and such.
Horseshoe crabs in the wild grow to well over a foot, a size I don't believe I have EVER heard them reaching in our reef tanks. The temperature is relatively high, the sand bed relatively small, and the current too strong. However, they can do very well in shallow, wide tanks with lots of sand and few rocks...lower temperature...specifically designed for them. Fascinating creatures for sure...we actually owe them alot...but not suitable for long term care in a reef tank. Nearly all are doomed in the long run.