AquaMaxx GFO Phosphate Out Granular Ferrous Oxide Filter Media - 2 Quarts 64 Fluid Ounces or 2 Quarts Rinse before use. Use 1 gram per gallon for saltwater aquariums or 1 gram per 2 gallons for freshwater aquariums. Test phosphate levels and replace media when levels begin to rise (approximately every 1-3 months). As with all ferric oxide media, alkalinity levels may drop when used. Be sure to monitor alkalinity and adjust as needed.