BULK  GAC - ESV's Granular Activated Carbon (GAC) based on bituminous coal provides the best combination of pore size, large particle size, high surface area, low ash, and hardness for chemically filtering aquarium water. In addition, test results indicate no leaching of phosphates. The high hardness level of this carbon prevents the dispersion of particles throughout the aquarium which can occur with softer grades of carbon. The large particle size facilitates water movement through the carbon and helps prevent the removal of planktonic food organisms via mechanical filtration. Usage: 1/2 cup per 40 gallons (1 cup per 40 gallons for tanks without protein skimmers) replace monthly.