[tang daddy]

As everyone has said the sand sifter is too large for a tank your size but if you want to keep it just know that it may starve to death..... The shell you found of the crab may be a molt as crabs do that often like shrimps shed to grow.

so you may still have a crab in your tank if you ever get a chance to see it make sure it doesn't get too big as some will eventually prey on fish!