you should either go with a shallow 1 inch or a deep 6 inches of substrate. I would suggest a fine grain sugar sand or caribsea aragonite reef sand is fine though. The reason for the two varied depths is that a shallow bed needs to be cleaned and turned over but isn't deep enough to become anaerobic. The deep sand bed will begin working converting excess to nitrogen gas. This is a good thing and needs to be deep enough to ensure this anaerobic area is not disturbed by snails and digging fish. Here is a good substrate site
DeepSandBeds