A gravel bed will reduce the chances of ‘channeling’ in the resin bed, whereby hard water flows through openings between the resin beads without undergoing the ion-exchange softening process. Every water softener supplier will have a different opinion on whether a gravel bed is necessary, and most household systems are sold without gravel. Generally, gravel is used with larger capacity systems of 2cu.ft. (64,000 grains) or more.