A 10 gallon tank is a good size. Hermit crabs like to be in colonies, so get at least 3. They need a substrate they can burrow into. There are a few other things to keep in mind to make sure they stay healthy: 1) they need a bowl of saltwater as well as a freshwater drinking supply; 2) their enclosure has to be humid -- they breathe through gills and will die if they dry out; 3) Keep the tank in a warm room or make sure the tank has some form of heat. They like it 75 degrees or warmer (but not direct sunlight or heat lamps). Good luck!