An underdrain system for a filter having filter media. The underdrain system preferably includes a bottom formed of concrete having a plurality of generally cone-shaped depressions formed therein. Fluid flows through the plurality of generally cone-shaped depressions during filtration and washing. The underdrain system further includes a fluid distributor operably associated with at least one of the generally cone-shaped depressions. The fluid distributor preferably has a plurality of elongated slots for distributing the flow of fluid passing into and out of the generally cone-shaped depressions. The slots are sized to prevent filter media supported by the fluid distributor from passing therethrough. An optional insert may be provided to ensure even distribution of fluid through the fluid distributor.