The filter cartridges from the flow cartridge system use phosphate-storing materials to quickly and effectively remove algae-causing phosphates from the pond water. The phosphate content is lowered below the growth limit for algae thus preventing the growth of algae. An increase in phosphates, for example, due to a bath in the swimming pond or natural pool, are collected immediately. Ultra-smooth glass beads on both sides and the matched cartridge design distribute the water flow around the centrally filled granulate and ensure an optimal storage result.