A screen apparatus for improved separation of particulates from a fluid stream. The screen is configured and arranged to filter relatively large particulates from the fluid stream as the fluid stream passes from a storage chamber of the tank to a tank outlet. The screen is removably affixed to a diverter, such as a baffle, used to divert a portion or all of the fluid into the storage chamber. The screen is preferably shaped to substantially conform to the shape of the diverter to aid in the smoothing of the fluid flow within the retention section of the tank. The screen may be entirely porous, or it may be non-porous at its leading edge to minimize particulate build-up at the transition from the tank interior wall to the screen. The apparatus may optionally include a second screen to provide additional filtering of floating particulates as the fluid level in the tank exceeds the crown elevation of the inlet. The second screen may be substantially horizontally oriented and may be combined with a collection weir. Alternatively, the second screen may be conical in shape. A high flow bypass arrangement for the separation system may include an upper baffle and a lower baffle or a screen instead of the lower baffle. The upper baffle is set closer to the center of the tank than the lower baffle or screen, thereby providing more volume for flow diversion behind the upper baffle.