I would say 1000 gal minimum, horse shoe crabs are not only dependent on the live sand to survive but a small male is 18 to 20 inches long and females are even bigger, an 8 gal tank is not appropriate for them at all. Even though they come into the trade at very small (one inch or less) size they feed off the live sand and a small tank can't support the amount of food they need to survive and grow. You can add food to supplement their diet, but then you run into the fact that if you care for the crab you will get very fast growth, then you are in the same position, they are out growing your tank and you'll either pollute your water or need to move it to a much bigger tank. In reality they are not really suitable for aquariums.