Bioballs don't do anything to aid in the breakdown of NO3
(your friend is right). The bacteria responsible for denitrification (the breakdown of NO3
back to N) need to be located in an anaerobic environment (i.e. no oxygen). Since the bioballs are constantly being washed over with fresh water carrying oxygen, these kinds of bacteria cannot live. However, bioballs are extremely efficient and letting the bacteria that convert NH3
and then NO2
thrive. This is because these bacteria need an aerobic environment (i.e. high oxygen).
I have no idea what the "nitrate reducers" are. I tend to not keep up with most of that "mumbo jumbo stuff" as I think it is the trade taking advantage of us fellow hobbyist. There are are only two ways to get rid of NO3
: nutrient export (i.e water changes or macro algae nutrient uptake) and denitrification which I outlined above. Perhaps those "nitrate reducers" are balls just like bio-balls but they are made to be put in an anaerobic environment.