Yes, you need a gravel be that is slightly higher than the distributor basket at to bottom of the riser tube. This is important as it lets the water--during the backwashing stag--spread out across the bottom of the Calcite and will make it lift more evenly and thoroughly.
Without it, you will notioce no diffrence at first, but in time the Calcite will start to clump and 'channeling' will occur causing the service to become poorer.
What make of valve do you have and what is the size (HxW) of your tank?
Andy Christensen, CWS-II
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