Unlike EI which provides plenty of everything there are other dosing schemes that try to go lean on dosing. Such approaches work for some. One way to do things is to adjust your dosing based on the results of test kits. What if we use a NO3 test as a way to adjust our dosing? Is there the potential to run into problems?
Let us use the K – NO3 combination as they are usually dosed together. Obviously, one can extend the issue to everything else that is dosed based on NO3 alone. It also works both ways, you can over- or under-dose. Furthermore, K is not very accurately measured in freshwater with hobbyist kits.
Sources of nitrogen
Popularized by Barr’s Estimative Index (EI), the addition of nitrate is frequently done by adding KNO3. This is a very elegant solution as plants use both K+ and NO3-. There are other sources of nitrogen that are easily accessible to aquarists such as Ca(NO3)2 or urea, but they have limitations. Nitrates might also be found in tap water.
Another source of nitrate often overlooked by aquarists is the biological activity of the aquarium. Fish food and metabolic activity of all the living things in the aquarium will (indirectly) add some nitrate to the water. The main sources of nitrate are urea from fish waste and amino acids. This is the reason why aquariums with few plants, lots of fish and few water changes will have high nitrates.
A simple model
To better understand the problem let us create a quick model (demonstration purposes only).
To simplify things, we will limit the model to:
- rate of uptake remains constant
- nitrogen comes only from tap, fish food (100% conversion) and KNO3 (100% purity)
- KNO3 is the only source of K
While this does not reflect the variability of a natural system, I think this is quite conservative and keeps the largest factors in focus .
The parameters of the model will be:
- KNO3 will be added to maintain a concentration of NO3 at 10mg/L
- Tap water has 3 mg/L
- 3 mg/L per week NO3 is added from fish food
- 2 mg/L NO3 and 0.7 mg/L K are removed (plant uptake etc.) per week
- 50% water change every 4 weeks with tap water
As tap water comes with 3 mg/L, we will add 7.36 mg/L from KNO3. This means 12mg/L KNO3,4.64 mg/L K+ and 7.36mg/L NO3-.
Average nitrogen content in proteins is 16%. A fish food manufacturer reports crude protein as 46%. This means for 1g fish food you add 73,6 mg N, the equivalent of 326,32 mg NO3. Assume we have some very hungry fish in 100L and we feed 1g per week. This means we add 3 mg/L per week NO3 to the system.
You could play with the rates of uptake and percentages, but sooner or later… This is just because a lot more nitrogen comes from things that are not considered fertilizers.
As you see with time you will have less and less K while maintaining a relatively good range for NO3. As mentioned in the introduction, this may happen with other nutrients when we dose them based on the result of another parameter.
What can you do to avoid it ?
Adjust the dosing so you have enough of K, or whatever the element with the smallest concentration is.
Frequent water changes and adding all nutrients to target levels prevents extreme values and large deviations from your targets.