While these are known to de-nitrate slowly, the load of the aquarium (number and size of fish) will ultimately decide if the nitrates can be broken down quickly enough. The best way is to do a water change on a normal basis, and for that its much easier and definitely healthier to invest in a capable RO system for the house and a holding tank.
Still I assume we are all versed in this so we'll skip that RO part.
De-nitrators rely mostly on anaerobic bacteria that feeds on nitrate and the volume and propagation of these bacteria strands are slow and easily upset. The most effective solution seems to be done with very fine sand, and deep substrate of over 6 inches. Reefkeepers use deep sand beds effectively in sumps and inside the tank, but for most freshwater aquarists who keep a heavier load usually cleans gravel on a regular basis...which is detrimental to de-nitrating bacteria (and if things go wrong, it can get very unsightly with detritus collecting at the shallow depths of gravel.) It's sort of a catch 22 with these systems, where one just needs to have a lot of dead space (I mean huge) that trickles water extremely slowly to allow this to happen.
In a tube reactor such as tubes of very slow flow rate, (I mean a few drips per minute -- slow) this can also happen and can be effective with a small load. Imagine though, that the fish waste created has to be slower than the drips of water through such system! That's SLOW...
However, to filter a heavily stocked aquarium, you'd better setup a big tank as a sump of deep sand bed and filter water slowly through it. This is the principle behind the "mud" sump filtration in fresh and saltwater tank. It's rather involved and takes a lot of patience to make it work. But it can work, when designed with proper volume, and with careful monitoring and setup until things start to rock and roll inside.
patience though, people prevail...most of us don't have it, and look for magical machines that make life easier. Sometimes science will work, and make life easier, other times it just causes more issues. When such systems become more efficient, and become a part of every recommended aquarium as wet/dry filters have today... we'll live in a better world.
