Hi Sims, could I suggest that you put something next to the snails when you photograph them to give an indication of their size. Viper is correct about the one in photo 2 - these live in the upper reaches and above of the high tide level so they will not stay in your tank. Where did you collect the one in the last photo? Was it in sand?
It looks like what is commonly called a plough snail (Bullia sp.). These live in the swash zone and bury themselves by ploughing back into the sand. They are essentailly the vultures of the swash zone and eat pretty much anything that they find from jelly fish to dead fish.
They like lots of water movement so I am not too sure how they would do in a low flow sceanrio such as a DSB. I would also be concerned that they would keep disturbing the surface sediments (probably top 2 to 3cms) of the DSB which is also not too great an idea.