12-23-2009, 08:19 PM
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Others have all correctly said it, but just to add my agreement, it is the plants coupled with a relatively small fish load.
Plants require nitrogen, and their preferred source for it is from ammonium. In acidic water the ammonia produced by the fish and biological actions changes to ammonium and the plants grab it. In basic (alkaline) water, the plants use the ammonia and "convert" it through their cell structure into ammonium, then they use the ammonium to photosynthesize. If you suddenly removed all the plants, you would actually have quite a mini-cycle because the bacteria simply are not there when there are live plants. The bacteria exist at a level sufficient to consume the ammonia, and because there is so little ammonia available because of the plants, the level of nitrifying bacteria (nitrosomonas and nitrospira) in a well-planted aquarium is very, very minimal. Plants are quicker at grabbing the ammonia/ammonium than the bacteria can become established. It takes something like nine hours for nitrosomonas bacteria to multiply when ammonia increases, which they do by binary division, and the plants have simply grabbed the ammonia/ammonium before this can occur.
With the low level of nitrosomonas bacteria due to the low level of ammonia/ammonium present, nitrite is also low and nitrospira bacteria similar. It is believed by many that plants also use nitrite by converting it back to ammonium. And the nitrite that does get converted to nitrate is minimal, and some species of plants will use the nitrate as well. Studies have shown however, that the majority of aquatic plants prefer ammonium over nitrate, and by a significant margin.
The reason I mentioned your fish load is because with fewer fish in the aquarium, less ammonia is being produced, and the plants are quite possibly consuming all of it. I have constant nitrate readings of 5-10 ppm in my aquaria; I suspect I see this level (though low) simply because I have a heavy fish load. With fewer fish I would expect the nitrate to be 0-5 ppm.
Just a last word on the API nitrate test kit. It apparently does give false readings usually on the high side if the regeant #2 is not shaken for at least 2 minutes rather than the 30 seconds mentioned in the instructions. I've come across situations both on this forum and elsewhere that substantiate this fact, so I would suggest that you shake the regeant for 2 minutes in order to get a more accurate result.