I used mud in my fuge i have about 1 1/2g of mud covered with about an inch of sand. when i setup my fuge this was what was suggested by John at Reef Cleaners. he said something about the mud being better for growing bacteria then sand, but you needed the sand on top to hold the mud and filter the the saltwater from the mud. the nutrients in the mud are better for the fuge and you have more directions for different types of biological filtration in the long run. (he also suggested mangroves witch i haven't got yet).