Nitrates too low
I've just tested my nitrates before doing my bi-weekly WC. The test gave me a result somewhere between 0ppm and 5ppm. Im using the API brand of test kit which is fairly new. I know for sure that it is still good because I also tested water for nitrates from a small container with decayed/decaying organic matter that I use to culture mosquit larva. This test gave me all very high readings on ammonia, nitrite and nitrate.
The tank is low tech with mineralized soil substrate, no ferts, no CO2, and not heavily planted. Fauna: a 2 inch angelfish(stunted-long story), 10 neons, 4 zebra danios, and a few small quilted melania. I do bi-weekly water changes summing up to 20% a month.
What could be causing such low nitrate levels?
18 gallon low-tech planted community