When I was first starting out with saltwater, me and my ma had a 30 gallon tank and one day when we were at the fish store my mom bought a little horseshoe crab that had to be alil smaller than an inch long. We put it in the tank and saw it maybe once. It buried itself and we never saw it again. After having an internship at the Aquarium in Charleston, SC I learned a lot more about the guys. They do get huge, they have indeterminate growth, so as long as they live, they grow and if under the right conditions can live for decades. They have to be one of the most interesting creatures out there. While it's small a 10 gallon would be able to hold it for a limited time, but after a few sheds you would definitely have to upgrade. I pretty much became an expert on these guys while I worked at the aquarium so if you got any questions I can probably help.