I.d. ?

[joesox]

I have been seeing this critter in my tank for about a year and a half now and was finally able to get a decent picture of it. I don't know if it is some kind of worm or a slug? Its underside is flat, kind of like a snail, and it only comes out at night. This picture was taken using the flash with the tank lights off. I have never seen it come all of the way out of its hole, but have seen about four inches of it. Any ideas?

 

[fshhub]

hard to see for sure
but does it have a small shell, flat one almost like a fingernail?
if so, I would say stomatella, they are god snails and detrivores that primarilly work on the rocks

 

[joesox]

No it doesn't have any shell at all. It just stays real close to the hole in the rock where it lives. I am also pretty sure that it is harmless, besides that it will push away anything that I try to place by it's hole.