I laid new sand down in my aquarium that had been up for several years without taking anything out (fish, crabs, snails etc). I had no problem with anything but you may want to either lay a small amount at a time or do have the tank then a week later do the other. I just added the 30 extra pounds all at once with no problem. If all you have are the crabs I doubt it would cause any problems. Somebody else can chime in if I'm wrong. Also I had severe problems trying to keep an anenome after my tank was first up and running. I didn't have any luck until after it was up for about a year. From what I understan they also require fairly intense light to really survive and last long.