A watershed runoff treatment device is adapted to be used to convert a single-stage drain inlet vault (catch basin) into a two-stage clarifier in which treatment can be accomplished before discharging the water to further on-site treatment or detention or to storm drain. It includes a flow control box member to be mounted on the interior of the vault covering the discharge pipe(s) so that substantially all water runoff entering the vault must flow therethrough prior to exiting the vault. Floating, disposable hydrocarbon collection pads are deployed inside an open mesh container removably attached and covering the upstream face of the flow control box, as well as below the horizontal filter array described next. A horizontal filter array with removable, cleanable filter media is deployed below the flow control box for collection of suspended solids gravity-settling within the vault space, and enabling standing water in the vault interior to gravity flow through the permeable filter and suspended solids and automatically discharge from the vault, thus providing mosquito control and de-watering of solids for easy removal.