I have a 10 gallon coral and invert QT that has been up for almost a year now but just added sand and more LR from the DT about 2 months ago. I added the sand and LR because I got a pistol shrimp, a bunch of snails and some emeralds and wanted them to have lots to eat and places to hide. All went fine, had a couple emeralds die on me. But when I went to remove the pistol shrimp I noticed the parts of the rock that were covered in sand turned a black/grey color. Anyone know what caused this? Any action I should take? Tank just has 4 conch and 2 emeralds now, the conchs go in on Saturday but the emeralds still have another 20 days. In the picture I have set the rock on it's side to show the bottom.