I've seen these guys periodically in my tank for years. They're small, around 1/2" to 3/4" long, seem to be mostly on the glass or rocks, and have a little brown bit of shell at the very back of their tail. I haven't been able to get a better picture than this, but their proboscis has darker splotches on the top, so I believe they're a kind of whelk. They haven't caused any problems that I'm aware of. I've seen opinions ranging from "they're fine if you don't have clams" to "kill it with fire!", though, so I would at least like to determine the species to know if there are particular incompatibilities I should know about. I'm pretty certain that they breed in the tank, as I find shells on occasion (as do the hermit crabs 
). I've never been aware of more than 3 or 4 at any given time, though, so not getting out of control.
). I've never been aware of more than 3 or 4 at any given time, though, so not getting out of control.
