You must have alot of stem plants. I have had times when I would put 1 gram in a 46 gallon and measure zero nitrates the next day. I've also noticed that 1 gram of KNO3 doesn't disappear as quickly if the plants have had a major trim recently. So maybe plants can suck up that much nitrate. I'm sure that a plant mass weighing and calculation could be used to verify but, that would be to much work.
