*ahem*
I am no SW guru, but this:
One possibly strange thing is they are all three laying in the same area of the tank, near rubble pile.....
Has some significance.
Is it just me that thinks to check under any rocks near by? Could there be a large crab hidden? or a mantis? If they are all in a pile, look for a small hole/entrance type thing in the sand or near any rocks...
i doubt all the shells would be in the same place if it were the water perams..
I could be wrong, but it seems logical..
James