Dead Snails

I've got two snails on the bottom of the tank that lost most of the colour
in their shells. I don't know if they were sucked out of their shells by
someone or if they died from a lack of minerals? Is the loss of colour an
indication of a mineral problem or would they lose their colour after dying
anyway? I suppose the Excel might have done them in too, but the other
smaller snails seem OK. (So far).


TIA