I ordered a finger sponge from naples sea shell co along with my shells last week. I figured it would be a fun thing for the crabs to climb on. I soaked it in salt water to kill any bacteria and then laid it out to the shape I wanted to dry before adding it to the tank. After I put it in I think every crab who wasn't under molting suddenly became very active (this was daytime). I noticed that it gives off a faint beachy smell in the tank. Do you suppose they like the smell?