For Softeners and Filter Systems Provides even flow for more effective filtration and backwash Low Cost Provides backwash dispersion at a low cost Product Description Gravel is commonly used to bed softeners and filter tanks. It is commonly used in standard density filter systems (such as carbon and Filter-Ag) where the density of the gravel is sufficent to maintain bed separation. If replacing the underbedding on denser filter systems (such as iron filters) please use garnet. Required amounts for underbedding common tank sizes: 8" - 10" Tank - 16 lb 12" Tank - 20 lb 13" - 36 lb 14" - 50 lb