I just realized the S210T 21" filter is only about $35 more than the 18". Hmmmmm?? More thinking. The flow rate for the 21" is 44 GPM at 20 GPM/sq.ft and 27 GPM at 12 GPM/sq.ft. And I saw somewhere that the 1 HP 2 speed pump puts out about 40 GPM @ 25' of head - on low. Is the profit margin really that much higher on motors than filters? Or do the filter people think they'll sell more other stuff if they sell you a terribly mismatched system?