Not meaning to discount anything that has been said here by anyone & certainly never wanting to appear as if I'm just trying to 1 up someone, I just want to make sure we're all on the same page. 
Fluvals fx5's manual (http://www.hagen.com/pdf/aquatic/FX5_user_manual_EN.pdf) says it's rated at 925 GPH @ zero head , loaded correctly w/ media, the manual states, a filter circulation of 607 gph, but notes "that flow rates were measured w/ intake & output hoses of the same length & without media " & does not give a gph # after a max head run allowed, so there are just a few more factor's to consider then just the highest gph # offered from any brand filter. So if we go w/ the 5/8" id tubing listed of Fluval's, a max hd run of 16'.4" allowed, w/ max media loaded, my very fuzzy math has me @ an actual returned gph filtered # of less than 500 gph. I'm sure someone w/ a better math sense for these figure's could steer us a bit closer, but I don't think I'm that far off. Oddball, Sir, maybe if you would, a little edumacation would be just what the engineer ordered, Sir.
I'm in no way am downing Fluval, in 25+ yrs. I have never owned anything other than Fluval in regards to CF's & for the most part, say 95% or so of the time, I have been extremely happy w/ them. Of the 5% of the time I wasn't happy w/ them, probly 2-3% of that was really my fault & not the products, i.e. failing to maintain them properly. It's easy to slack off maintenance of them when they seem invincible, lol. As a credit to them, since I stopped slacking on my maintenance, I have maintained 5 of them, all of which are still running after 3-12+ yrs. I have def got my $$$ worth & will continue to purchase them in the future w/ out a doubt.
Fluvals fx5's manual (http://www.hagen.com/pdf/aquatic/FX5_user_manual_EN.pdf) says it's rated at 925 GPH @ zero head , loaded correctly w/ media, the manual states, a filter circulation of 607 gph, but notes "that flow rates were measured w/ intake & output hoses of the same length & without media " & does not give a gph # after a max head run allowed, so there are just a few more factor's to consider then just the highest gph # offered from any brand filter. So if we go w/ the 5/8" id tubing listed of Fluval's, a max hd run of 16'.4" allowed, w/ max media loaded, my very fuzzy math has me @ an actual returned gph filtered # of less than 500 gph. I'm sure someone w/ a better math sense for these figure's could steer us a bit closer, but I don't think I'm that far off. Oddball, Sir, maybe if you would, a little edumacation would be just what the engineer ordered, Sir. I'm in no way am downing Fluval, in 25+ yrs. I have never owned anything other than Fluval in regards to CF's & for the most part, say 95% or so of the time, I have been extremely happy w/ them. Of the 5% of the time I wasn't happy w/ them, probly 2-3% of that was really my fault & not the products, i.e. failing to maintain them properly. It's easy to slack off maintenance of them when they seem invincible, lol. As a credit to them, since I stopped slacking on my maintenance, I have maintained 5 of them, all of which are still running after 3-12+ yrs. I have def got my $$$ worth & will continue to purchase them in the future w/ out a doubt.
