have several mushroom rocks in the tank - two doing perfectly. But the big rock which had about 12 blue-purple shrooms is now - within two days - totally nude. Not a single mushroom on it. Not on the sand either - nowhere to be found. What in the world could be eating them? Have the normal clean up crew and no bad hitchikers that I know of. Help! (no before and after photos - sorry.) Everything else in tank is just fine. thanks