Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Nitrate in an aquarium can come from 2 main sources:
Organic, such as fish food, or dying plants
Inorganic, such as fertilizer, KNO3 for example.
Stop feeding for a few days. Protein in the food contains nitrogen, and this is turned into nitrates through a series of organisms including the fish, decomposer bacteria and nitrifying bacteria. Thoroughly clean the tank and filter. Debris trapped by the filter is still in the system, and decomposing to form, among other things, nitrate.
Read the labels on all the fertilizer and water treatments you are using. Make sure none of them are adding nitrogen in any form to the system.
NO3 levels in a planted tank will vary according to how fast the plants are using whatever nitrogen (in any form) that is present.
In a tank with fast growing plants they can remove all the nitrogen pretty fast, and the NO3 will test zero. This is not good. Suggests the plants are right at the edge of deficiency.
Dosing KNO3 can raise the NO3 level, but the fish do not seem to be bothered by high NO3 from this source. Plants can then remove this pretty fast.
I generally allow the NO3 to vary from 5-10 ppm, and dose if it gets lower, or do a water change if it gets over 20 ppm. I think high tech tanks are dosed to a higher level, but the plants grab the NO3 so fast that I do not think those tanks will test very high, except right after dosing.