not quite lets say you go buy and overflow or drill the tank and get one rated at 100 Gallons per hour, in this case your pump must be under 100 GPH thats accounting head pressure, when you get the pump it will be rated for 200 GPH at 0' head pressure and at 4' 150 and 100 at 6' (or something like that, the pump will have a chart) so your pump has to be under 100GPH or your tank will overflow (bad
) so make sure they match up, as your overflow should always be more then your pump