First off the FX6 is an awesome filter but if you already have an FX5 then there is really no need to upgrade.
The main changes are color aesthetics and the fasteners used to be rubber.
However, if you are looking for a new filter and the price for the FX5 & FX6 are within the same range, then I would suggest you go with the FX6.
In my opinion, it’s the best canister filter made right now, if you have the money and you want a filter that does everything itself then the FX6 is the way to go.
Is The Fluval FX6 Quiet?
For being one of the largest canister filters with a huge pump the Fluval FX6 is very quiet. As with most filters and powered equipment, there is a low humming that is only detected when close to the aquarium.
Seamless Water Changes (Update July 6, 2016):
Fluval recently updated the FX6 & FX4 with a built-in water change system, multi-stage filtration, enhanced media capacity and even an optional powered FX Gravel Vac with independent filtering capability.
Check out this video showing all the new features of the Fluval FX6 & FX4
From Fluval Aquatics