Since switching to citric acid stop and low-odour rapid fix in a Nova I rarely bother to switch my fans on (I still usually run the developer in an open dish 'cos I like to see the magic happen...). Just the breeze blowing the air through the light trap is sufficient.

However, I can open a window when toning or otherwise using chemicals with an odour so if you do not have a window you can open in the basement you may need more serious forced ventilation. The extractor should, as others have pointed out, be behind and a little above the trays - biased towards the fixer end as that seems to be the most likely source of pongs. Mine are at one end of the sink 'cos that's where the window is and I was too lazy to run trunking...

As I say, I find I need very little forced ventilation as long as I can open a window for the post-processing operations with some toners and I am quite sensitive to fixer fumes which used to give me a two-day sore throat if the ventilation was not working well. Now with low-odour fixer (Fotospeed FX30 in my case but there are others) in the Nova I have zero problems even with the fans switched off. A testament to both low-odour fixer and the Nova methinks...

Have fun... Bob.