PhosGuard™ rapidly removes phosphate and silicate from marine and freshwater aquaria. Although PhosGuard™ is effective in both fresh and marine water, it will operate better in sea water and the need for it is better defined in sea water. It is not recommended for phosphate buffered freshwater (PhosGuard™ would remove the buffer until saturated). PhosGuard™ is highly porous and bead-shaped to promote easy water flow through and around it, thereby increasing both its efficiency and capacity.Sizes: 100 mL, 250 mL, 500 mL, 1 L, 2 L, 4 LComparing Seachem Phosphate RemoversProductPO4 boundPhosGuard (Alumina Oxide)0.87 mg/gPhosBond (GFO and Alumina Oxide hybrid)1.19 mg/gPhosNet (GFO)1.41 mg/g