piles of sand

[ornum]

hi
i have a hitchiker mantis that used to live in holes in the rockwork quite happily, it was about 2 inches long and about 2-3 months ago it disapeared.

since then ive been getting little piles of sand around the base of rocks but when i blow them away with a baster nothing is there.

could the mantis be doing this? there are crabs in the tank as well but no fish

 

[ornum]

i should add that my sand is quite large particle wise, there are small piles away from the rocks as well but these seem to be made up of sugar fine sand where as the pile next to the rock is not.

so it appears something is grinding up the sand in places, any ideas what?
thanks

 

[BodiBuilt]

Do you have any urchin in your tank?

 

[ornum]

no urchin, could be some kind of worm though