you must also take head pressure into account
jsut as an example
say i was filtering.....mmmmmm......a 240! if i used a RIO 32HF thats rated at 1920GPH, it is actually that at 1ft Head pressure, but say the pump is 5 ft below the tank, then that will go down to about 1300 gph, still pretty good but some pump lose more head pressure the farther they are from the tank.
jsut as an example
say i was filtering.....mmmmmm......a 240! if i used a RIO 32HF thats rated at 1920GPH, it is actually that at 1ft Head pressure, but say the pump is 5 ft below the tank, then that will go down to about 1300 gph, still pretty good but some pump lose more head pressure the farther they are from the tank.
