Fx4 could be overkill, If you were planning to run an inline heater, co2 reactor or any other inline device, it might be a valid option. You can always dial back the flow on the canister itself.
If you are running just the filter I may suggest a 407. I tend to like a bit more flow than most too, so take this with a grain of salt.

 

Generally you want 4 to 8 times turn over of the aquarium volume per hour. A 40 breeder actually has almost 50 gallons of water in it by dimension. AND canister filters only produce half of their estimated turn over per hour in the aquarium. So the FX4 is actually right on the upper end of acceptable to use. Certainly it will be stronger flow then many filters but can still work for you if you want to use it.

 

Correct.

Fluval list two figures for their FX series filters: pump output and filter circulation.

The listed filter circulation for an FX4 is 450gph. Using an unused tank as a water reservoir, I timed how long it took to fill a 5 gallon bucket. Based on that, I got a flow rate north of 400gph, with the top of each container 5’ off the ground. This is a cycled filter, with stock foam media + 30ppi Poret center foam block.


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