For a tank that is 48 inches by 18 inches, you probably want about 25 kilograms of sand or gravel to get a nice thick layer. If you don't expect to ever plant anything and just want a thin layer on the bottom you could get away with about 10 Kg to just cover the glass. All sand and gravel, whether from the pet shop or the home improvement center, will need to be very well washed or the dirt will form a cloudiness in your tank as soon as you add water.