To obtain the sand of a water tank having an alkali degree and a hydrogen ion concentration suitable for an organism to be reared, having a property capable of controlling the electric conductivity of the water, and further having a function capable of maintaining an artificially made water quality.

This sand contains calcium in an amount of 23-56% converted into CaO and silicon in an amount of 0.01-35% converted into SiO2, and has a thermal reduction amount of 18-44%, a whole alkali degree of 30-45mg/liter, and a phenolphthalein alkali degree of 1.5-5.0mg/liter. Or, the sand contains calcium in an amount of 0.1-5.0% converted into CaO, silicon in an amount of 70-97% converted into SiO2, aluminum in an amount of 0.0-10.0% converted into Al2O3, iron in an amount of 0.0-4.0% converted into Fe2O3, a thermal reduction amount of 0.01-4.0%, an electric conductivity of 5-35μS, and a whole alkali degree of 5-20mg/liter.