PhosGuard rapidly removes phosphate and silicate from marine and freshwater aquaria. It is not recommended for phosphate buffered freshwater. PhosGuard is highly porous for high capacity and bead-shaped for optimum water flow. It outperforms all competing products. 500 mL easily treats over 150 gallons*. This product is sold by volume. Cited weight is minimal weight. Sizes: 100 mL bagged, 250 mL, 500 mL, 1 L, 2 L, 4 L, 20 L, 100 L