My tank has been setup for 5 months and I just noticed it on the sand bed. It looks to have made a shell out of some live rock. The only live rock I've added was one pound 2 months after setup. The rest is dry rock. Is this a limpet snail? Good addition to have?
it's the shell of a whelk. I can't see if there's a crab in it or a whelk though. Meanwhile, some are ok and some are not. Easy enough to remove if it is a bad guy though. Just keep an eye on him as he grows.