Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Have you used root tablets? (DIY or commercial)?
The major sources of nitrogen in an aquarium are fish food, fertilizer and 'other' perhaps the tap water has nitrogen, or some other odd sources.
If it is fish food, then you would see a fairly smooth rise through the week, then a drop with a water change.
If it is root tablets they usually do not last more than several weeks to a couple of months, but you might see a spike of nitrogen if something disturbs the substrate.
Similarly, if something in the substrate is releasing nitrogen (perhaps decomposing organic matter) then there will be a burst of ammonia, nitrite or nitrate depending on how active the microorganisms are.
If the nitrogen is coming in with the tap water in the form of ammonia (such as chloramines, or other) there would be spikes of ammonia the day of the water change, nitrite a day or so later, then nitrate.
If the tap water has nitrate, then you would not see ammonia or nitrite spikes, just the nitrate test would give odd results after a water change, such as: if you did a 50% water change, and the tap water had zero nitrates, then the nitrates in the tank would be cut in half. But NO3 in the tap water would skew this, and the NO3 test after a water change would not show 50% reduction.
I would test the tap water, then set up a regular schedule of larger water changes until you can get the NO3 down to 10ppm or less on the day of a water change, then allow it to rise to 20ppm right before a water change.
Do what you can toward vacuuming the substrate, at least the top few grains, to lift out fallen food, leaves and so on. If you cannot reach the substrate, then you could use a power head the way a gardener uses a leaf blower. Clean the filter.