the widest part of the filter is the lid. and that is 14.5 inches across. so it looks like it will fit. might be a tight fit, so you might need to widen the entrance a little like you said. the box thing you see in the picture is the pump for the filter. i just measured mine and its 16 inches tall(not including the valves) and 14.5 wide(lid) the bottom is is 10inches across and the pump adds 4 inches to the side that its on. i think the fc5 will fit for sure. it also comes with a large booklet and dvd for setup so you wont be confused on how to set it up.