Aquarium filters and I suspect pool filters are flow rated when empty. Why empty? Well how can you standardize the effects of the media in the filter, across the board. Will a Fluval filter with its media have the same flow rate with an Eheim with its media? So to equalize the equation, flow rates are calculated with a filter empty.
I know in one source I go to helping me make decisions on my tank, the Eheim pro 3 2073 is rated at 62% of its flow capacity when the "fresh" media is in, and its slows down after the media gets "older." One respected authority on the net I respect flogs pond filters as being excellent and good value for the money.
Personally I wouldn't use sand as my filter media, too tight, but instead use a media such as Seachem's Matrix, something that will encourage massive "good" bacteria.