Well, they do come in as hitchhikers. That being said, I don't know the stages of growth and WHEN they manufacture their tube. But i can imagine they come in and finish off their growth while in the tank. I tried to download a few images, but only one made it onto my phone.
Does it have anything (like two antennae-looking things, for example) sticking out of it on occasion?I know I've seen them before, and I don't think it's a vermetid snail. Can't get a good angle on it, but it's just a hard, smooth black stick with some kind of build up at the base, almost as if it were a frag stuck in pea sized mound of putty.
