I am pretty sure that hob whisper filter denitrates the water via the
cartridge it uses. It is pretty common for filters to control
ammonia/nitrates/nitrites with a cartridge containing something like
zeolite that absorbs such things. However my impression is if you use
such measures you're committed to doing so until a major change takes
place with the tank. Tank cycling involves good bacteria eating ammonia
and nitrates, reproducing, and populating surface area aerobically (the
good bugs need oxygen) so if you take away the ammonia etc. then those
good bacteria are greatly reduced if not eliminated. Then, if you start
not using cartridges or replacing them in other words then fish will
probably die off because the tank will start cycling again now that
ammonia et. is there and not absorbed by the zeolite or whatever. So in
the long run it is best to not use such cartridges and cycle the tank
before more than 1 or 2 fish are in the tank. Otherwise you just have
to keep replacing cartridges for the life of the tank setup. Hope I've
helped and if I'm wrong someone please step up and point out what they
want. Later!