That absolutely looks like a mini-strombus to me. I just purchased some from Indo-Pacific Sea Farms, and they look identical.
The telltale to me though is your last shot. He's looks like he's trying to eat algae off the bottom of the plastic. That's grazing snail behavior. A predatory snail just sits tight or cruises looking for prey.
Sorry for the dark pic, but the one in the foreground I call the "blonde" variety. Their shells are usually a blonde or yellow color that's mostly solid, with a darker cone.
Both varieties can exhibit a clear or lighter colored foot and some will have a reddish hue.
All whelks will have a operculum, but nassarius, strombus and the like don't have that protection.
I have to flip out the cages on my gyres this weekend, so I should find some egg cases, then I'll post a pic.