Hey Paul
I have had quite a few tanks now with a varied filter systems... from the cheap to the expensive and i have come to the conclusion that they are just not powerfull enough. depending on the size of your tank (mine 1000l & 600l & 350l & 200l) i am replacing the filter system on my large 1000l with a pond filer system. For the same price as a fluval FX5 (which only pumps around 1300l (so it says on the tin but i dont believe that at all) i can get 1500l of filtration per hour from a blagdon force hybrid 16,000l + fishmate 15,000l external canister. i already have a blagdon 24w UV filter and my tanks are as clear as gin!
like i said - you need a good size tank to be able to hide the pump but i just dont see the reason why i shouldnt be doing it any other way.
hope this helps...