i have one in my 55g, they are sand sifters. good for a CUC. mine stays hidden burried in the sand most of the time, but occasionally i will see him catching a current and swimming around my tank. they generally require large tanks, but get one small and you should be okay for a while
Don't do it.
They get huge (several feet) and are very difficult
Exerpt from Wet Web Media
They are also said to be able to suffer seasonal famine and live without food for as much as a year! A year is also about how long it takes for these crabs to starve in mismatched marine aquariums. Too often, horseshoe crabs are placed in small aquaria with lots of live rock and not enough sand to burrow, forage and survive. Mind you that an abundance of live rock is very beneficial for reef aquariums. The problem is that these creatures, though, do not live on the reef proper. They do live near the reef, however, and will thank you very kindly if you provide a large open bed of deep fine sand for them to dig in. It takes perhaps 10 sq. ft per crab of more at >6"/15cm depth). It is interesting to note that horseshoe crabs may stay buried in the sand for days or even several weeks at a time. Let's be very clear that they live or die in captivity by the presence of deep fine sand and mud and the food they find within. We recommend using finely minced clams and clean Tubifex worms (live or thawed frozen) as part of the staple diet for this creature in captivity. Other ocean meats may be taken with equal enthusiasm. Be very mindful of the size and composition of food offered; Limulus have no jaws to chew or crush prey but instead process food with bristles at the base of their walking legs (yes... they must walk to feed, as the food is passed and mashed by these bristles). Although hardy and peaceful in their natural habitat, these "crabs" do not fare well in captivity outside of large, mature specialized aquaria. They tend to linger for some months in captivity, but again, do not survive past a few months with casual care.
Great piece of information Lynn.
I have tried to keep a couple of these alive in the many years in this hobby only to have them die after a few months. Now I admire them in the ocean where I think they should stay.
I have kepted my pair for about 3 years now, the most important things I have come across with keeping them healthy is you must feed them high protein and calcium based diets to support their molting and the temperature itself, 72 to 75 has been the best temp to keep them with some other fish that can handle the colder water, they generally die in a reef from my personal experience being do to the water temperature itself being above 75 degrees for a long period of time (and these are specifically from the gulf of Mexico, so said the store owner) I also run my salinity around 1.025 - 1.027
They generally will hide in the sand for very long periods of time after they have a larger feeding, but they do come out at night around 3 am to explore and move to a new place to hide.
I highly recommend that if you keep these goofy characters that you introduce a lot of diverse sand burrowing worms and crustaceans (generally living on live rock) and you arrange your aquarium in a way so that they can't get stuck between rocks or the glass and a rock as these guys cram themselves into tight spaces sometimes seemingly for no reason.
Mine have grown from being 2 inches from nose to tail tip to almost 4 - 5 inches, they will grow to be over 2 feet but generally that process will take several years, from my research almost 10 to reach full size and i personally look forward to it and have myself prepared they molt very rarely as well and when they do it can be a surprise since the shell looks just like they crab itself (if it goes smoothly) and can appear as if your crab has passed but the first thing to check for to make sure it isn't the case is a split in the front of the shell where they exit out.
I feed my crabs a mix of reef frenzy (frozen) Hikari massivore delite and Hikari crab cuisine with occasional raw shrimp and they have been doing very well with this mix so far.
It's not possible to keep them in a traditional reef tank and they generally prefer colder waters but if you supply the a lot of wide deep sand to live in and you keep the rock layout safe for them, you can keep them. I will also say it's very important for them once they start growing to supply square foot space over water amount, they need wide open spaces to be able to move about and do random crab things, for younger crabs under 6 inches I recommend the 55 low boy tanks as they supply a lot of space for them.
hopefully this information is helpful to anyone trying to keep horseshoe crabs in a home aquarium!