What is the most effective depth of gravel for using an underground filter? TIA Chuck Church CEFChurch at aol_com Indianapolis, Indiana 46206-2067 USA ><((((º>`·.¸¸.·´¯`·...¸><((((º>¸.·´¯`·.¸ .·´¯`·.. ><((((º>`·.¸¸.·´¯`·...¸><((((º> ·.. ><((((º>`·.¸¸.·´¯`·...¸><((((º>·´¯`· . .·´¯`·.. ><((((º>