Gravel is okay to use in the water bowl section of the tank, but don't use a lot as it is hard and not good for digging. I wouldn't use a lot of calci sand either because they don't like to dig in it and it clumps when wet. Also, it is possible for it to get stuck in a hermit crabs shell and harden, trapping them! You should take it out, put it in a plastic baggie and use it as food instead. It is a great calcium source so I often sprinkle it in the food dish. You should replace the calci sand with play sand or coconut fiber (my favorite) so they have something else to dig in just in case the saw dust isn't good. Remember, this is a happy forum and we're all trying to help!