My conchs go through the sub.
I believe Nassarius are sand sifters also..They smell food and hit the surface of substrate like a submarine breaching water !
The Nassarius are small around 1/2 inch so dig fine tunnels and stir in little ways ..
My conchs are 5years old and about 2-3 inches..They bulldoze the substrate when they want ,but also spend a lot of time on top in plane view..I think they are too cool with their elephant trunk and eyes...
My experience with the Nassarius is they stay under unless there is food above .